THE FUNDAMENTAL PRINCIPLES OF PETROLOGY McGraw-Hill BookCompany Electrical World The Engineering and Mining Journal En5ineering Record Engineering News Railway Age G azette American Machinist Signal Engineer American Engineer Electric Railway Journal Coal Age Metallurgical and Chemical Engineering P o we r THE -FUND AMENTAL PRINCIPLES OF PETROLOGY- BY DR. ERNST WEINSCHENK PROFESSOR OF PETROGRAPHY IN THE UNIVERSITY OF MUNICH AUTHORIZED TRANSLATION (FROM THE THIRD GERMAN EDITION) BY ALBERT JOHANNSEN, PH. D. ASSOCIATE PROFESSOR OF PETROLOGY IN THE UNIVERSITY OF CHICAGO WITH 137 FIGURES AND 6 PLATES FIRST EL-IT ION McGRAW-HILL BOOK COMPANY, INC, 239 WEST 39TH STREET. NEW YORK LONDON: HILL PUBLISHING CO., LTD. 6 & 8 BOUVERIE ST., E.G. 1916 Q & COPYRIGHT, 1916, BY THE MCGRAW-HILL BOOK COMPANY, INC. THE. MAPLE. PRESS* YORK. PA Oo U)e of Caroline Austin this translation is ~2>r}. juera:juop(/>6o//a:i, to be transformed). When the appearance of a rock is due to later metamorphism, all conclusions as to the original process of formation, drawn from this obviously secondary characteristic, are necessarily incorrect. Petrographic investigation of the crystalline schists has shown the following fundamental principle of petrology to be correct: The petrographic characteristics of a rock have no relation whatever to its geologic age. The earliest sediments must have had a uniform character, for they were derived from and deposited upon a universal, uniform, preexisting crust, and while it is probable that the oldest deposits resembled certain of the crystalline schists in many ways, yet the latter do not possess the necessary homogeneity. From our present petrographic knowledge we can consider it only very improbable that the true, primitive crust is accessible anywhere upon the earth's surface. III. VULCANISM AND THE ORIGIN OF IGNEOUS ROCKS LITERATURE W. BRANCA: "Schwabens 125 Vulkanembryonen." Stuttgart, 1894. Idem: "Vulkane und Spalten." Comptes Rendes X congr. geol. internal., 1906, 985. W. BRANCA AND E. FRASS: "Das vulkanische Ries bei Nordlingen." Abh. Akad. Wiss., Berlin, 1901. Idem: "Das kryptovulkanische Becken von Steinheim." Ibidem, 1905. H. CREDNER: "Elemente der Geologic," II Aufl., 1912. R. A. DALY: "Abyssal Igneous Injection as a Causal Condition and as an Effect of Mountain Building." Amer. Jour. Sd., XXII (1906), 195. C. DOLTER: "Die Petrogenesis." Braunschweig, 1906. Idem: " Physikalisch-chemische Mineralogie." Leipzig, 1905. ARCH. GEIKIE: "The Ancient Volcanoes of Great Britain." London, 1897. G. K. GILBERT: "Report on the Geology of the Henry Mountains." Washington, 1877. A. HARKER: "The Natural History of Igneous Rocks." London, 1909. A. LACROEX: "La montagne Pelee et ses eruptions." Paris, 1904. P. SCHWAHN: " Die physikalische Grundlage der Stubelschen Vulkantheorie." Him- mel und Erde, 1a/j,is, force, fjieTa/j.op(K, Li) 2 O + Na 2 O. (a) After adding Li to K, allot A1 2 O 3 to all the (Li, K) 2 O and all the Na 2 O in the proportions of one to one to form (K, Li) 2 O-Al 2 O 3 and Na 2 O-Al 2 O 3 . Let A represent the number of molecules so formed, that is A = (K, Li) 2 + Na 2 0. (6) Allotment of remaining A1 2 O 3 . If A1 2 O 3 >(K, Li) 2 O -+ Na 2 O but less than CaO + (K, Li) 2 O + Na 2 O, the re- maining A1 2 O 3 , after 6a is satisfied, is united in equal proportions with CaO to form CaO-Al 2 O 3 . The number of molecules thus formed is represented by C. That is, C = A1 2 3 - A. (c) If Al 2 O 3 >CaO + (K, Li) 2 O + Na 2 O, add BaO and SrO to CaO and unite in equal proportions with A1 2 O 3 . If any A1 2 O 3 still remains unused, take enough (Mg, Fe)O to satisfy it, and add this to C as a molecule of (Mg, Fe)O-Al 2 O 3 . 7. A1 2 O 3 < (K, Li) 2 O + Na 2 O. (a) Take an amount of Fe 2 O 3 equal to the remaining alkali (amount of excess of alkalies over A1 2 O 3 ) to form the molecule (K, Li) 2 O-Fe 2 O 3 , and add this, as aegirite molecules, to A. The numerical value of A still remains A = (K, Li) 2 O + Na 2 O. (6) Excess of alkalies. n A1 2 O 3 + Fe 2 O 3 <(K, Li) 2 O + Na 2 O, the excess of alkalies is to be added to A as a separate group, so that A contains the total alkalies as before. 8. The sum of all the molecules of MnO and NiO, and of the molecules of CaO not used in rule 66, and the molecules of FeO, MgO, SrO, and BaO not used in rule 6c, is represented by F. That is, F = (CaO + FeO + MnO + MgO + NiO + SrO + BaO) - C. The Fe 2 O 3 not used in rule 7 is recalculated as FeO and is added here. 9. Recalculate Na 2 O plus (K, Li) 2 O to 10, and indicate the value of Na 2 O thus obtained by n. That is, 10Na 2 O 10Na 2 O n = (K, Li) 2 + Na 2 10. Recalculate the sum of A, C, and F to 20, and represent these proportions, to the nearest 0.5, by o, c, and/. a + c + / = 20. 11. The rock-formula now may be written in terms of s, a, c, /, and n, as shown below. It may also be plotted on a triangular diagram whose vertices are designated by 20a, 20c, and 20/. The rock is indicated by the point where the three lines, which represent these values, cross (Fig. 43). Two other values are used occasionally in certain rocks, but they are not so important as s, A (a), C(c), F(f), and n. They are as follows: 12. Recompute (Mg, Fe, Mn, Ni)O and (Ca, Ba, Sr)O as used in F, so that their 70 FUNDAMENTAL PRINCIPLES OF PETROLOGY sum equals 10, and let m represent the value of (Mg, Fe, Mn, Ni)O thus obtained. That is, _ 10(Mg, Fe, Mn, Ni)O = (Mg, Fe, Mn Ni)O + (Ca, Ba, Sr)O ' The silica coefficient (/c) has the following value: k = 6A+2C +F As a check on the work we have s + 2A + 2C + F = 100. For rapid calculation, Kaiser used the following additional formulae. After com- puting the molecular proportions and reducing to 100, he determined N = 100 - (s + A1 2 O 3 + P 2 O 5 ), p = 4 A1 2 3 V- N ' From which a = 20P, c = 20(Q - P), / = 20(1 - Q). These formulae, with the essential formulae of Osann, s = SiO 2 + ZrO 2 + TiO 2 , A = (K, Li) 2 O + Na 2 O, 10Na 2 O A ' n = are all that are necessary for a computation. In the above formulae A (a) represents fairly accurately the amount of alkali- feldspar and feldspathoids in the rock; nephelite, leucite, sodalite, hauynite, the segirite molecule, the potash molecule in mica, and the alkali molecule of aenigmatite and eucolite here being considered feldspathoids. No definite amount of SiOa is added to A to form these minerals from the alkalies, and they do not necessarily occur as such in the rock. To determine the minerals accurately one must know the relative amounts of the individual constituents and their actual compositions. The variable amounts of alkali in the micas and in the feldspathoids make the conversion difficult. In the case where Al 2 Oa<(K, Li) 2 O + Na 2 0, part of the alkali unites with iron oxide to form alkali-bearing pyroxenes or amphiboles. This is usually the case only in rocks which are alkali-rich and which contain feldspathoids. In these rocks, naturally, C equals zero, and the projection points lie on the A-F line of the diagram. C(c) represents the amount of the anorthite molecule in the plagioclase. In subordinate amounts the aluminium-bearing molecules of pyroxene and amphibole are contained here, and, in melilite rocks, the gehlenite molecule also. F(f) represents the relative amounts of the dark constituents. It contains the AUOs-free and the alkali-free pyroxene and amphibole molecules, the olivine molecule in olivine and mica, the akermanite molecule in melilite, the iron content of the iron ores, and the lime content of apatite and titanite. n gives broadly the ratio of the orthoclase to the albite, or of the potash feld- spathoid to the soda-feldspathoid. k is the silica coefficient, and shows the ratio of SiO 2 to A, C, and F. To obtain THE COMPOSITION OF IGNEOUS ROCKS 71 k, therefore, the value s must be divided by a number equal to the sum of the silica molecules necessary to change A and C to feldspars and F to the metasilicate. According to this system (see Fig. 43 and rule 11) a rock may be represented by a formula similar to the following : S59 Os-5 C12-5/4 /l8-3- This indicates a rock in which there is 59 per cent, (molecular) of SiO 2 , while (Na, K) r A1 2 O 4 : CaAl 2 O 4 : (Fe, Mg, Ca)O = 3.5: 12.5: 4, that is, its plagioclase is rather rich in lime and forms (3.5 + 12.5 = )16 parts of the 20 into which the rock was divided, or 80 per cent, of the whole. The basic constituents constitute the other 20 per cent. We also have the relation Na 2 O:K 2 O = 8.3:1.7, that is, soda is much more abundant than potash, which was to be expected in a plagioclase rock. The rock is a gabbro-diorite. Numerous graphic methods for representing the chemical compositions of rocks have been devised, but only the simplest will be given here. In a diagram either the FezOs AhOa KjO NazO CaO ^^~. ~..~. FIG. 40. Graphical representation of the composition of a granite from Hauzenberg, near Passau. (After Rosenbusch.) analysis recalculated to 100 per cent., the molecular per cent., or the metal atom per cent, may be used. For example, in a parallelogram 10 cm. long ( = 100 per cent.) percentages may be shown in millimeters, the difference in the proportions in different rocks giving figures which enable one, with a little practice, to obtain a very good impression of the composition of the rock. Thus a granite from Hauzenberg near Passau is shown in this manner in Fig. 40. The upper rectangle represents the weight percentage, the middle the molecular percentage, and the lower the metal-atom per- centage corresponding to the analysis below. This is recalculated to 100 per cent., water and the minor constituents being disregarded. II III SiO 2 73 83 82 1 70.4 A1 2 O 3 12.30 8.1 13.9 Fe 2 O 3 .... 4 18 1 7 2.9 CaO 94 1 1 1 Na 2 O 2.22 2.3 3.9 K 2 O 6 53 4 7 7.9 Totals.. . 100.00 100.0 100.0 Another diagram for representing rock analyses, suggested by Michel-LeVy and modified by Brogger, consists of four axes radiating from a center (Figs. 41 and 72 FUNDAMENTAL PRINCIPLES OF PETROLOGY 42). The SiO 2 content is plotted in millimeters upon the horizontal line, one-half to the right and one-half to the left of the center. The other constituents are indicated by proper intercepts on the other axes, the points so determined being connected SiO* FIG. 41. Graphical representation of the composition of the granite from Hauzen- berg, near Passau. (After Michel-L6vy.) by lines. Fig. 41 represents the percentage weights in the Hauzenberg granite, Fig. 42 those in a gabbro from Radautal i. H. As mentioned in rule 11 above, the rock-types computed by Osann's method may be indicated by points in a triangle, the apices representing 20a, 20c, and 20f. If CaO MgO FIG. 42. Graphical representation of the composition of a gabbro. (After Michel-Levy.) the values of a, c, and / are laid off upon the medial lines, a point is obtained for each rock of definite chemical composition. The example given on page 71, 03.5, 012.5, /-4, is shown by the dot in Fig. 43. FIG. 43. Graphical representation of a gabbro-diorite. (After Osann.) The system of Cross, Iddings, Pirsson, and Washington, in which the chemical types and textures of rocks are represented by stereotyped "index words," is much too complicated and voluminous to be given here, even in outline. V. ROCK WEATHERING LITERATURE See the bibliography given by JOH. WALTHER: "Lithogenesis der Gegenwart." Jena, 1894. M. BAUER: "Beitrage zur Geologic der Seychellen, insbesondere zur Kenntnis des Laterits." Neues Jahrb., 1898, II, 163. G. BISCHOP: "Lehrbuch der chemischen und physikalischen Geologic." 4 vols., Bonn, 1847-1855. 'A. DAUBREE: "Les eaux souterraines de 1'epoque actuelle." Paris, 1887. A. HEIM: "Handbuch der Gletcherkunde." Stuttgart, 1885. E.JW. HOFFMANN: "tlber den Einfluss gewohnlichen Wassers auf Silikate." Leipzig, 1882. T. STERRY HUNT: "The Decay of Rocks in Mineral Physiology and Physiography." Boston, 1886. A. JOHNSTONE: "On the Action of Pure Water and Water Saturated with Carbonic Acid Gas on the Minerals of the Mica Family." Quart. Jour. Geol. Soc., London, XLV (1889), 363. GEO. P. MERRILL: "Rocks, Rock-weathering, and Soils." New York, 1906. A. PENCK: "Morphologic der Erdoberflache." Leipzig, 1894. F. V. RICHTHOFEN: "Fiihrer fur Forschungsreisende." Berlin, 1901. H. ROSLER: "Beitrage zur Kenntnis einiger Kaolinlagerstatten." Neues Jahrb., B.B. XV (1902), 231. J. ROTH: "Allgemeine und chemische Geologic." Bd. I, "Bildung und Umbildung der Mineralien." Berlin, 1879. I. C.; RUSSELL: "Subaerial Decay of Rocks and the Origin of the Red Color of Certain Formations." Bull. 52, U. S. G. S., Washington, 1889. P.'TREITZ: "Was ist Verwitterung." C.R.I, confer, intern. agrogSol., 1901, 131. J. M. VAN BEMMELEN: "Les divers modes de decomposition des roches silicatees dans la croute terrestre." Arch, neerl. des sciences exactes, XV (1910) (2), 284. Weathering in General. The term weathering is used to de- scribe all alterations brought about by the atmosphere and the agents present in it, and by organisms at the surface and within the lithosphere. The action of weathering is in part chemical and in part mechanical. It primarily destroys the original rocks, whose constituents, after more or less separation by wind and water, are finally deposited to form the sedimentary rocks. Weathering acts as a leveler of the relief of the earth, and is con- fined to the parts adjacent to the surface. The term replacement, on the other hand, includes alterations not due to the action of the atmospheric agents. The changes, therefore, do not begin at the surface, and are not confined to the 73 74 FUNDAMENTAL PRINCIPLES OF PETROLOGY upper strata. Other changes are produced by diagenesis (Gr. 5id, after, yevevis origin), or the re-formation by the hydrosphere of the newly deposited products of weathering, during and directly after the sedimentation. The action of the atmospheric agents is chiefly regional. It is dependent upon the moisture content and temperature of the air, as well as upon the content of chemically active agents. The latter are usually uniformly distributed but may be more abun- dant locally, for example near the sea or near recently active vol- canoes. Also, certain agents derived from organisms are active, either by their life-processes or by their decay. There are to be distingushed, therefore, physical, chemical, and organic weathering. In most cases all of these processes act together. Physical Weathering. The phenomena of physical weathering are seen best in arid regions, where insolation (Lat. in, in, sol, sun) and great temperature differences break compact rocks into great blocks and cause the exfoliation of granite and sandstone. The great sand mass, so characteristic of the desert, is due primarily to the different behavior of different minerals under temperature changes. While chemical weathering is much more active than physical weathering in damp, tropical climates, the latter distinctly pre- dominates in the temperate and frigid zones. In the temperate zone the water circulating in the capillaries of the rocks is an impor- tant factor in their destruction. In fact the amount of water absorbed by a rock is taken into consideration in determining its commercial value. For example, the freezing of water in porous sandstones, limestones with clayey layers, etc., may make them friable, or may even totally destroy them. Physical weathering is especially active in high mountain regions, on account of the occurrence of great variations in temperature. Chemical Weathering. Chemical weathering is usually accom- panied by physical and organic weathering, and is dependent upon moisture and temperature. Where the evaporation of the rainfall is greatly retarded by a thick cover of vegetation, as in the tropics, it is of great importance. Water falling through the atmosphere takes up certain chemi- cally active substances even from the purest air of high mountains, and while these substances are generally present in small amounts, ROCK WEATHERING 75 they are of great importance. Besides the predominating content of oxygen, CO 2 as well as traces of HC1 and H 2 SO 4 are always pres- ent, and even after the water has passed through fresh, sulphur- free silicate rocks, a certain amount of chlorides and sulphates still remains. Geologists have been inclined to over-estimate greatly the importance of chemical weathering, and all the products of rock-decomposition have been continually con- founded with weathering products, rendering very difficult a general conception of the complete process of rock alteration. Certain rocks undoubtedly have been more or less completely dissolved by weathering. Thus, rock salt occurs on the surface FIG. 44. Karst topography. Wiesalpe, Dachstein. (After F. Simony.) only in arid regions, and the so-called "gypsum chimneys" are simply pits dissolved in fractured gypsum-rock. Limestone also is more or less easily soluble, and the water from melting snow or glacial ice, in some cases, corrodes the underlying rock until its surface resembles a stormy sea turned to stone (Fig. 44). Further, certain peculiar funnel-shaped depressions of the Karst, called dolinas, belong here, and the accumula- tions of red clay, terra rossa, which occur within them are doubtless the residues from the dissolved limestone. "Geologic pipe organs" have a similar origin. These are vertical cylinders which occur in limestones, especially where the surface above was heavily forested. Recent investigations make it appear very doubtful whether all large limestone caves are due to solution. Such caves usually occur in coral limestones, and it is worthy of note that entirely analogous openings are found in recent coral masses where they represent original gaps in the reef. Like the limestone caves, the bottoms of these gaps are covered with terra rossa or similar cave-loam. Coral islands in general show such deposits, consisting of drifted-in laterite and weathered volcanic ashes and pumiceous sands. The presence of these floor deposits appears to be an argument against leaching action, for moving water would have carried away the fine mud of 76 FUNDAMENTAL PRINCIPLES OF PETROLOGY which they are composed. Stalactites certainly indicate extensive solution, but they tend rather to close up large caverns than to produce them. In open spaces the dis- solving power of water is lessened by evaporation, loss of carbonic acid, and the action of organisms, giving rise to the formation of sinter. All caverns in limestone, there- fore, cannot be ascribed to atmospheric agencies, although some undoubtedly represent widened fissures. The term chemical weathering, in the strict sense, is less commonly applied to the solution of entire rock-complexes than to the alteration by atmospheric agents of the primary rock-form- ing minerals. By its action, sterile rock is changed to fruitful, arable land, and its study is the foundation for the study of the soil. Chemical weathering is due primarily to the action of vadose waters, and since the activity of the latter depends especially upon climatic conditions, chemical weathering, in general, depends upon climate. It is, therefore, as has already been mentioned, a regional phenomenon. Under similar climatic conditions, the same kinds of rocks everywhere produce approximately the same kinds of weathering products. It is hardly necessary to point out that chemical weathering is more intense in moist, warm climates than in those that are dry and cold, but the nature of the action and the new minerals produced differ but slightly. Mineralogists and geologists have long been accustomed to ascribe practically all alterations of minerals to vadose waters, and this is especially true of the alteration of anhydrous minerals to hydrous. A closer examination, however, shows that numer- ous complicated processes which have been ascribed to atmospheric agents, actually are always intimately connected with volcanic phenomena, and that certain altera- tions ascribed to weathering are simply local and not regional occurrences. For example, one of the most important of these non-regional alteration processes which has been incorrectly ascribed to weathering, is the formation of kaolin from feldspar rocks. A close examination will show a series of phenomena proving the incorrectness of former conceptions as to its origin. Kaolin always appears in very irregular, isolated masses surrounded by normally weathered granite. It is usually white and forms a marked contrast to the rust-like products of weathering. Further- more, it is nearly or entirely free from potash, and always entirely free from apatite. The salts essential to plant growth being thus wanting, kaolin soil is extremely unpro- ductive, differing markedly from the soil from weathered granite, which is very fertile. It is true that certain local conditions may produce a much more intense leaching than usual, whereby the residual deposits will differ from the ordinary rusty materials. Such is the case in the alterations produced in rocks by overlying peat bogs, whose organic acids, acting as reducing agents, carry away in solution all of the iron content. In this manner white weathering products, in many cases very kaolin-like in ap- pearance, are formed, and these, without careful examination, might be identified as kaolin. The fact that the granite in the more important deposits is completely kao- linized to depths far beyond that reached by the atmospheric agents under any circumstance, and that the deposits nearly always contain new minerals (Ger. Neu- ROCK WEATHERING 77 bildungeri) such as tourmaline, topaz, fluorite, scapolite, pyrite, and siderite, which certainly do not normally crystallize from vadose water, is disregarded; as is the fact that bog-weathering never produces aggregates having the composition and properties of kaolin. Hereby we come to a fundamental difference between the vari- ous products of rock alteration. Kaolin and analogous products of decomposition are crystalloids, while the chief weathering prod- ucts of feldspars and other silicates are unquestionably colloids or gels. So far as is determinable with certainty, normal weather- ing products, in the main, are amorphous, and the property of the soil to adsorb the alkalies and the alkaline earths with great ease, and just as readily give them up again to plants, is a prop- erty characteristic of colloidal substances and marks an especially important difference between the products of weathering and kaolin. Finally, if plants could remove all of the alkali and lime from a soil, leaving only a pure aluminium silicate, it could later regain these salts by taking them from solutions; kaolin does not possess such a power of adsorption. Because the alkalies and alkaline earths of zeolites may be replaced in part, the conception of " soil-zeolites " was introduced in 'soil-study, but although true zeolites crystallize very readily, none that was newly formed has ever been observed in a normal soil, nor has the probability of the existence of such crystalloids ever been shown by chemical or other investigations. Substances which in the mineralogic sense may properly be called zeolites are as rare among normal weathering products as in crystallized kaolin, and the term " soil-zeolite" should be avoided, for it pro- duces misconceptions in regard to chemical weathering. The constituents of the soil which have been so designated are, without doubt, true colloids. Great confusion likewise has been produced in the study of weathering by the double meaning given to the word day. Under this term are included secondary deposits of more or less pure kaolin as well as the finer products of true weathering. The latter material, which was brought together by running water, consists principally of the colloidal products from weathered silicates and of fragments of quartz. As is to be expected from the manner of their origin, kaolin clays are purely local occurrences which have originated from the destruction of primary kaolin deposits. The clays of weathering, on the other hand, are normal, widely distributed, regional sediments which usually differ greatly 78 FUNDAMENTAL PRINCIPLES OF PETROLOGY from true kaolin in chemical composition and mineralogical character. Finally, as the last straw, the term laterite (Lat. later, tile), which is properly applied to the normal red weathering product of the tropics, has also been used for another peculiar, secondary substance of the tropics. The latter material is composed chiefly of colloidal hydroxides of alumina and iron, and thus differs entirely from normal laterite, whose chemical composition is not greatly different from that of the original rock. From such misconcep- tions originated the view that kaolin represents the end-product of weathering in temperate and cold regions, while bauxite is the end-product in the tropics, where the warmer atmospheric agents had much greater chemical energy. The principal effect of chemical weathering is the destruction of the rocks. There are formed (1) a soluble part the weathering solution which carries off the basic constituents of the minerals by the aid of the acids, especially carbonic acid, which are always present in vadose water; and (2) a weathered residue, remaining where the weathering took place. Since nothing is lost in the cycle of katamorphism and anamorphism, the weathered products of the primary rocks must appear again somewhere in the secondary. Thus the average composition of argillites and sandstones nearly corresponds to that of the weathered residuum of granite. The solids of the weathering solutions are deposited elsewhere, the cal- cium carbonate contents being almost entirely separated by organ- isms, while the remaining solids appear in chemical sediments such as those in the rock-salt formations. If kaolin is due to weathering, great quantities of potash twice as much as of soda should have been leached from regions which have been kaolinized, for about 90 to 95 per cent, of the primary rocks are of the composition of granite. As a matter of fact, potash-salts in sediments derived from weathering solutions are very rare as compared with those of sodium. If the bauxite of the tropics were produced by the weathering of granite, approxi- mately 7 per cent, of potash and 40 per cent, of silica, mainly from feldspars, must have been carried away. Such great quanti- ties of potash are not found in the precipitates from the solutions, and even less can the enormous masses of dissolved silica be found. It is true that deposits of silica, separated from weathering solu- tions by the action of organisms, are known in all formations, ROCK WEATHERING 79 but they are exceptional occurrences and rarely reach any consid- erable magnitude. If the normal product of tropical weathering were bauxite, however, two-fifths of all sediments should consist of silica derived from the solutions of weathering by organic or chemical precipitation. That this is contrary to the actual facts was shown in the chapter on the formation of sediments. The Weathering Solutions. The material leached from the rocks by vadose waters is found in springs, brooks, streams, seas, and oceans, and from the composition of these waters the action of chemical weathering can be best studied. Waters which fall through the atmosphere doubtless always carry, besides oxygen, certain dissolved acids, which are the primary agents of chemical weathering. In all running waters, besides predominant carbon- ates, there are present sulphates and chlorides whose acid radicals came from the atmospheric agents and whose bases came from the rocks through which the waters passed. Meteoric water, upon evaporation, shows hardly a trace of solid matter, but this is always present in variable amounts in springs, brooks, and streams. While the destruction of the rocks by these weak agents is very slow, locally the processes may act somewhat more intensely. The in- creased activity of the solutions when strengthened by organic acids has already been mentioned. The weathering is most active where the surface rocks are rich in minerals such as pyrite, whose oxidation sets free great amounts of sulphuric acid. Such weather- ing is especially characteristic in the gossan of ore deposits. The composition of weathering solutions varies greatly in different regions. Where the climate is humid the percentage of dissolved matter is less than in dry cli- mates on account of the greater evaporation in the latter. The salt-pans of the deserts are simply concentrated salt-solutions whose constituents were leached from the rocks, and whose water may all disappear during long dry periods. In areas of granitic rocks in humid climates, the vadose waters are especially poor in solids, the proportion sinking to 1 : 50,000; ordinarily the percentage in springs and streams is from ten to twenty times as great. Carbonates predominate in all of these solutions, the amount averaging 70 to 80 per cent, of the total residue. The remainder consists of about 10 to 15 per cent, of sulphates, 5 to 10 per cent, of chlorides, and a very small amount of silica, perhaps 0.5 per cent. Of the total dissolved material, 70 to 80 per cent, consists of calcium-salts except in regions which are entirely granitic, 10 to 15 per cent, of magnesium-salts, and 10 per cent, of sodium-salts. The potash contents is rarely greater than 0.5 per cent. In every case, the weathering solutions are very poor in silica and potash. The composition of surface waters becomes greatly modified in arid regions when the solid constituents are concentrated by rapid evaporation. Accompanying this concentration, in many cases, there is a great decrease in the calcium content, the carbonate being precipitated after becoming insoluble in the concentrated salts 6 80 FUNDAMENTAL PRINCIPLES OF PETROLOGY solution. Chlorides and sulphates, in rather varying proportions, are here of most importance, the Chelif River in Algeria, for example, containing as much as 7 gm. residue to the liter. Of this residue, carbonates form about 2 per cent., sulphates over 50 per cent., and chlorides 40 per cent., corresponding to 25 per cent, each of calcium- and magnesium-salts, and the remainder of sodium-salts. Here, likewise, silica and potash are of no importance. Evaporation increases the salt content of the true salt-seas of the steppes until the waters are saturated. Soda and magnesia are of prime importance, although they occur in decidedly variable proportions, while lime is relatively unimportant, having been almost entirely precipitated as gypsum. Bitter-seas, rich in magnesia, may con- tain as much as 30 per cent, of dissolved constituents. Other materials of weath- ering, such as the salts of strontium, which ordinarily are hardly perceptible, may occur in distinctly recognizable amounts. It is worthy of note that phosphate plays no role in vadose waters. "Practically all of the apatite of the original rocks remains in the weathered residue, from which it may be withdrawn later by vegetation. On the other hand, nitrates are here and there present. They form extensive deposits in the Chilian pampas, and represent a peculiar development of salt-pans. Their origin has been ascribed to frequent and tremendous electrical discharges which oxidized the nitrogen of the air. Borax-seas and borax deposits likewise occur in deserts, but they have a somewhat different origin. They appear to occur exclusively in volcanic regions where boron- bearing fumaroles have acted upon salt deposits. The not uncommon cementation of desert sediments with silica, however, is to be traced to vadose waters, whose small silica content was precipitated in the general concentration of the weathering solu- tions. By far the most silicified rocks, however, were altered by silica-rich juvenile waters, and not by solutions of weathering. Running waters carry their dissolved constituents to the ocean, and thus, by the continual addition of solutions, even though very dilute, and the evaporation of , pure water, they have gradually become perceptibly salt. Although the entering waters come almost exclusively from humid regions, the composition of the sea water differs entirely from the weathering solutions brought in. Most noteworthy is the almost complete loss of carbonates. These were partially removed by precipitation as oolite when the solution was sufficiently concentrated, but in the main they were deposited as organogenic sediments, having been taken up by organisms which required calcium for their hard parts. The normal residue of ocean water consists of not quite 80 per cent, sodium-salts, about 15 per cent, magnesium-salts, 5 per cent, calcium- salts, and 1 per cent, potash-salts. These are combined in the form of chlorides about 90 per cent., and sulphates 10 per cent. The chief constituent is sodium chloride, although in ordinary weathering solutions it is of slight importance. Exhaustive investigations of weathering solutions show with absolute certainty that normal weathering has never altered granite to kaolin, nor even to bauxite. This is in harmony with the conclusion already drawn from geologic relationships. The Weathered Residues. A study of the weathered residues leads to exactly the same result as did that of weathering solutions. This is shown by a comparison of the composition of a fresh granite, for example that from Altmittweida in Saxony (I), with that of the granite-grush 1 derived from it (II). 1 The translator has taken the liberty of using the verb grush, meaning to crumble down, to disintegrate, as a noun for the finely crumbled but not dust-like rock called grus by the Germans. ROCK WEATHERING 81 SiOt AhOs 1 Ft MgO CaO NasO KiO HiO I 73.43 14.38 2 19 0.22 j 0.68 : 3.03 6.07 96 II 73 84 14.62 1 ?,8 0.35 0.37 1.56 5 98 2 85 These analyses show that only lime and soda have been with- drawn in any considerable amount from the rock by the atmos- pheric agents. The increase in magnesia in the weathered product is probably an error in the determination. The noteworthy feature, however, is the fact that the proportions of silica, alumina, and water in the second analysis are entirely different from those in kaolin. Kaolinized granite contains at least one and one-half to two times as much alumina and about five times as much water as the granite-grush above. Granite-grush may be sorted later by wind and flowing water, and then be deposited as sandstone or calcareous argillite; the former consisting essentially of quartz, the latter of the col- loidal weathering products of the rock-forming silicates. The silica content is large in sandstones, and few conclusions can be drawn from these rocks in regard to the course of the weathering of the silicates. A study of the finer material which was washed out and deposited as argillite gives better results. For example, a very plastic kaolin from Klingenberg on the Main (I) and a nor- mal sedimentary clay from the English coal formation at Frank- land (II) have the following analyses : SiO 2 1 AhOs FezOs MgO CaO NazO K 2 HjO I 49 37 30 10 3 9 38 16 24 II 61 91 20 73 5 01 59 5 25 3 16 6 73 An inspection of these and the analyses previously given, shows without question that kaolin and bauxite are not normal weathering products. The same conclusion is reached from an examination of the chemical composition of the soils produced by organic weathering. The primary result of all weathering processes is the removal of the lime, magnesia, and soda by vadose waters. The greater part of the potash remains in the soil, to be very gradually withdrawn, though seldom entirely, by plants. The colloidal alteration products of the silicates which were formed by chemical weathering are to be recognized from the physical properties of the soil rather than by direct observation. These colloids form but very thin films upon the surfaces of the different minerals, so that a completely disaggregated granite-grush still permits the main features of its chemical composition to be clearly recognized. The potash of the feldspar is very slowly leached from the thinnest beds, and is made accessible to plants through colloidal aluminium silicates. The leached soil may be regenerated 82 FUNDAMENTAL PRINCIPLES OF PETROLOGY artificially through the addition of potassium-salts, which become absorbed by the colloids, or naturally by allowing the land to lie fallow. In the latter case the potash of the smallest feldspar particles in the soil is made available by the superficial altera- tion of this mineral to colloids, which later give up their content of alkalies and alkaline earths to plants. The slow process of true weathering is thus entirely different from the relatively rapid and radical process of kaolinization, and this difference is shown not only by the greater or lesser rapidity of the processes themselves, but by the nature of their end-products, kaolin and colloids. Climatic Zones of Weathering. The difference in the character of weathering in humid and dry climates was mentioned under the heading of weathering solutions. Here the influence of hot, tem- perate, or cold climates upon the final character of the weathered products will be described. Qualitatively the weathering solu- tions are the same in all zones, and the differences in the character of the products are very small. The external rather than the in- ternal characteristics of the weathered residues are of importance. The intensity of chemical weathering depends upon the hu- midity and temperature of the atmosphere. It acts rapidly and deeply in the humid climate of the tropics, but only superficially in the arctics. The differences in coloring in different climatic zones are especially noteworthy. In the polar regions the weath- ered products are light-colored to white, in the temperate zone they are typically rust-colored, while in the tropics they usually have the red-brown to red color of laterite. This difference is due primarily to the extreme slowness of weathering processes in cold climates, the oxygen of the vadose waters being extremely in- active here, and the iron oxide is removed from the rocks by solu- tions as fast as it is formed. In the temperate zone, the iron is precipitated as colloidal iron hydroxide or rust; the higher the temperature of the surroundings, the less water in the iron hydrates and the redder the color. That the intensity of the color of any weathered rock depends upon the percentage of iron in it, is self- evident, for example a gabbro or trap normally weathers darker than a granite, but the degree of color of any particular rock- type indicates its climatic zone. It may here again be mentioned that the name laterite is used in the tropics not only for normal weathering products, but also for peculiar deposits which consist primarily of colloidal aluminium and iron hydrates. While always local in occurrence, some of these deposits are of considerable size, and many of them grade into commercially valuable masses of bauxite and limonite. These, ROCK WEATHERING 83 however, as has been pointed out several times, are entirely anom- alous formations, and while in some cases they can be distinctly recognized as alteration products of different igneous rocks, they certainly cannot be considered to be their normal weathering prod- ucts. In many places analogous masses are found which un- doubtedly are new formations (Neubildungeri), being in part superficial incrustations, in part aggregates or fragments embedded in limestone. In their irregular form and their pisolitic texture, which in many cases is distinct, they resemble the bauxite deposits of the sediments of former geologic periods. In general, also, they differ from normal weathering products in the irregularity of their bedding. Laterite, the normal weathered material of the tropics, covers a considerable part of the earth's surface, and upon much of it there is a dense growth of vegetation. The anomalous deposits, however, have a different appearance, for heavy vegetation cannot grow from soil composed t)f pure aluminium hydrate and quartz, since the salts necessary for plant nourishment are wanting. The red color of the weathered material in the tropics is undoubtedly the result of a warm climate, and it is as justifiable to conclude from the red color of a sediment that a warm climate existed during its deposition, as it is from the presence of tropical flora and fauna. Similarly, very light-colored sediments generally indicate a former cold climate. The color of the weathered material in the different climatic zones, therefore, serves as an important aid in the study of the earth's history. Chemical Weathering of Former Periods. The course of chemical weathering, as it occurs at the surface of the earth at the present time, may be accurately followed. The question arises, may not the climatic conditions in former geologic periods have been so greatly different that ordinary atmospheric weather- ing yielded products which, at the present time, form only under very exceptional circumstances? The carbonic acid which is now stored in the limestones and dolomites, the carbon of organisms, and the chlorine which is present in the waters of the ocean, were all doubtless taken from the primordial atmosphere. Our present atmosphere contains about one-thirtieth of 1 per cent, of carbon dioxide, but even a very slight increase in the amount would make it impossible for the higher animals at least, to live. If all of the carbon dioxide of the limestones and dolomites and the chlo- rine of the chlorides had been contained in the original air, there would have been present, in far remote geologic periods, such enor- mous amounts of these agents, that it would have been neces- sary for the respiratory organs of animals to have been very differ- 84 FUNDAMENTAL PRINCIPLES OF PETROLOGY ently organized. Such an active atmosphere, also, probably would have caused intense alteration of the rocks, such as occurs only locally at present. The sequence of weathering can be followed in the oldest geologic periods with nearly the same accuracy as it can at the pres- ent time. The mechanical sediments show the composition of the residuals of weathering, while the chemical precipitates show at least the fundamentals of that of the weathering solutions. Both mechanical and chemical sediments, from the oldest Cambrian formations to the present, show the same characters throughout, consequently there is no reason to suppose that any notable change has taken place in the character of the atmosphere during that time. Thus there is an apparently unbridgeable difference between direct observation and speculation. But this difficulty vanishes when a factor, until now neglected, is taken into account, namely the influence of vulcanism in preserving the equilibrium in the composition of the atmosphere. The carbon dioxide, brought from the interior of the earth, continues to replace that which is being stored in the carbonates and other organic sediments. During periods of especially great volcanic activity there resulted an atmosphere high in carbon dioxide. This led to the development of unusually abundant vegeta- tion and the reestablishment of the equilibrium. The most extensive organic sedi- ments, therefore, are found in connection with formations which are characterized by exceptionally intense volcanic activity; for example, the Carboniferous and the Tertiary. From such observations the conclusion is justifiable that while the amount of carbon dioxide in the atmosphere varied during past geologic periods, the propor- tion was never very different from that existing at the present time. So far as can be determined from the formations remaining, there never was enough carbon dioxide in the air at one time to produce the limestone formations now found. In the same manner, the chlorine content of the atmosphere remained constant. Free hydrochloric acid is added by volcanic eruptions, the volcano Purace in Colombia : alone giving off over 30,000 kg. daily. Organic Weathering. The processes of organic weathering, which generally accompany and are usually intimately connected with the chemical processes, are somewhat different. Vegetation removes from the rocks, especially from the comminuted products of chemical weathering, substances which are not soluble in the atmospheric agents. Chemical and physical weathering make the constituents of the rocks more available to plants, and these, by the secretions of their roots and the cooperation of soil bacteria, extract the substances necessary to their existence primarily ROCK WEATHERING 85 potash, lime, and phosphoric acid. Although lichens, moss, and the like are able to obtain their nourishment directly from rocks, any considerable number of the higher plants can obtain the neces- sary salts only from the colloidal weathering products. Different plants withdraw the salts from the soil in different proportions, and thus, while they remove certain constituents, they produce, by the destruction of new material, an enrichment in substances less necessary to themselves. This fact is of great importance to agriculture. Where chemical weathering is slight, the nourishing salts of a soil are frequently exhausted. But this exhaustion is only apparent. When the fields lie fallow for a time, the necessary constituents are renewed during the period of predominating chemical weathering, and new parts of the soil are prepared for the use of the vegetation. Rock-sculpture by Weathering. Different kinds of rocks are affected by weathering in very different ways, and upon this de- pends, primarily, the relief and richness of form of mountains. Rocks which are rich in glass weather much more easily than those that are holocrystalline, and porous and schistose rocks and those that are internally crushed are much more easily destroyed than those that are compact. The alkali-rich silicates, especially nephelite and the minerals of the sodalite group, weather espe- cially easily, and in many cases are dissolved out entirely at the sur- face of the rocks. The weathered surfaces of nephelite-syenites, therefore, are usually pitted and corroded. In other cases, rocks which in the fresh state appeared homo- geneous throughout, show different susceptibilities in different parts, and irregularities, which originally were not visible, may be brought out distinctly by the action of weathering. The weath- ered stratum or alluvium (Lat. eluere, to leach), therefore, does not form a cover of uniform thickness parallel to the surface, -but an uneven layer; in fissures and over easily decomposed rocks the action has taken deep hold, over resistant rocks there is only a thin cover. The rock-sculptures produced by the strong air currents of the deserts differ entirely from those produced in a warm, humid, tropical region with dense vegetation. In deserts all decomposi- tion products are blown away, and the fresh rocks are exposed. In the tropics most of the rocks are porous and much altered to considerable depths, and in place of sharp cliffs, the forms are rounded, and fresh rocks seldom appear at the surface. There is likewise a characteristic difference between the weathered forms 86 FUNDAMENTAL PRINCIPLES OF PETROLOGY on medium and high mountains, the material on the latter being carried away as soon as it is formed. The characteristic partings and jointings of certain rocks FIG. 45. Platy weathering in granite. Rudolfstein, Fichtelgebirge. (see Sec. X) have considerable influence on the weathered forms. In many cases these joints are distinct in the fresh rocks and con- tinue downward unchanged, for example, in basaltic columns; FIG. 46. Rock stream. Reichenbach, Odenwald. (Prof, Dr. Klemm, photo.) in others they appear only in the weathered rocks and then are gradually lost with increasing depth. Jointing may be seen in many granites. At the surface the rocks have a more or less thick-platy parting (Fig. 45), but as the dis- ROCK WEATHERING 87 tance from the weathered surface increases, the joints are farther and farther apart (cf. Fig. 6), until finally homogeneous rock is reached. But even in this compact rock there are invisible parting-planes, called rift and grain, which are generally due to a parallel orientation of the minerals, and which permit easy cleavage in those directions. The rock is made porous along these planes by weathering, and the circulating water which enters brings out the superposed plates. A rock usually possesses several such jointing directions, so that atmospheric agents may round off angular fracture-surfaces, and produce spheroidal forms. If the weathered material between the harder parts is carried away, the rock may become so incoherent that apparently solid rocks are suddenly precipitated into so-called " rock-seas " or " rock- streams" (Fig. 46). FIG. 47. Spheroidal weathering of diabase. Fichtelberg, Fichtelgebirge. Dr. Klemm, photo.) (Prof. The weathering agencies tend to round the edges of rocks of uniform texture where they are cut by joints, and produce forms resembling loaves of bread. This is seen in the fractured diabase in Fig. 47, and in the jointed basaltic columns in Fig. 48. On the other hand, rocks which are internally mashed, as are the central Alpine granites, fall into sand and grush upon weathering. The thinner the laminae of schis- tose rocks, the more easily do they weather, consequently true schistosity assists weathering as much as does transverse schistosity. Many phonolites show a thin-platy parting at the surface (Fig. 49), while farther down they are compact but have a rift in parallel planes. The platy partings of cer- tain quartz porphyries, on the other hand, were not caused by weathering but by con- traction during cooling, which in many cases was so great that quartz phenocrysts were sheared across in the same manner as are olivine crystals hi basalt, parts of the same crystal occurring in adjacent columns. 88 FUNDAMENTAL PRINCIPLES OF PETROLOGY The first indication of weathering in a granite is generally the appearance of rusty infiltration products. These may be present, not only in the granite-grush, but even in the apparently fresh rocks. They penetrate from the surface downward, as if FIG. 48. Spheroidal weathering of basaltic columns. (J. Roth.) Schlossberg, Aussig, Bohemia. through fissures, and form a yellowish stain, indicating that the rock is porous, or appear in the form of dendrites (Gr. bkvbpov, tree) in the joints. If the weathering penetrates deeper than usual, as in the granite of the Bavarian Forest, there may be found, beneath the upper rusty layers, completely altered sandy beds, still showing the FIG. 49. Platy weathering of phonolite. Black Hills, S. D. (J. D. Irving.) original granitic texture and with still unoxidized iron, the waters having lost their oxygen before reaching this depth. Denudation. The forms produced by weathering are made apparent by the forces of denudation (Lat. denudare, make bare). ROCK WEATHERING 89 These, however, do not simply transport the weathered material, but they attack and wear away the country-rock by means of the fragments carried. The thinner the cover of vegetation, the stronger the denudation; therefore it is particularly great in deserts, on high mountains, and in the polar regions. The weathered forms of a denuded region differ according to the agents. Wind and continental ice denude the entire region over which they act, and are not dependent upon the slope of the surface as are flowing water and glacier ice. The denuding action of wind is called defla- tion (Lat. deflare, blow off), that of flowing water erosion (Lat. erodere, gnaw out), that of the sea abrasion (Lat. abradere, scratch off), and that of glaciers exardtion (Lat. exarare, plow out). FIG. 50. Corraded granite. Russian Turkestan. (Prof. Dr. Merzbacher, photo.) Sand masses moved by desert storms may wear away even the hardest rocks. This corrosion (Lat. corrado, scrape together) may gnaw deep holes in uniformly granu- lar rocks like granite (Fig. 50), or polish compact rocks, or pit those consisting of granular aggregates of minerals of different hardnesses. Furthermore, flat, arched facets may be ground on the larger fragments (Dreikanter} , or the surface may be gouged and smoothed until it suggests that of a meteorite. This resemblance is made still greater by a glistening black crust of manganese and iron hydroxides, the so-called desert varnish. The wind also concentrates the larger, harder constituents of broken rocks while it blows away the more easily destroyed interstitial parts. This is seen, for example, in the accumulations of flint nodules, fossils, etc.; the so-called desert pavements. The residual forms produced by the wind depend upon the character of the rock. Compact rocks form steep, high cliffs, sharp needles, and jagged peaks, while horizon- tally bedded rocks become flat table-lands with surfaces of harder strata. If these are cut by sharp fissures and cross-fractures, typical bad-land topography with characteristic mushroom forms, results (Fig. 51). 90 FUNDAMENTAL PRINCIPLES OF PETROLOGY FIG. 51. Bad-land topography. Washakie Basin, Wyoming. (After C. King.) FIG. 52. Bosses, Grimsel hospice. Bernese Oberland. (Photo. Photoglob.) ROCK WEATHERING 91 Denudation by continental ice, like that by wind, is regional, but the forms produced are entirely different. The whole weathered surface is removed by the FIG. 53. Grand Canyon of the Colorado. former, and the fresh rock takes on predominatingly rounded forms, the so-called roches moutonnees (Fig. 52). While fresh rock may be exposed by the sand-blasts of FIG. 54. Trass deposits. Tonnisstein, Brohltal, Eifel. (After Volzing.) the desert as well as by the plowing action of continental ice, the surface exposures are different. In one case they are covered by the peculiar desert varnish, in the other polished and striated by the scouring of the ground-moraine. In neither case are the 92 FUNDAMENTAL PRINCIPLES OF PETROLOGY new deposits confined to the valley bottoms; they also occur as sand-glaciers or moraines high up in the mountains. Glacial action is more local and is generally confined to valleys. It produces ero- sion forms analogous to those formed by continental ice. In each case there is a FIG. 55. Dolomite ridges. Bozen. tendency to round corners, smooth out the sharp and hard lines of the landscape, and widen the' valleys. $\ Denudation by means of flowing water is entirely different. Here everything moves downward, and deposition is confined to the valley bottoms. Where there FIG. 56. Silicified baryte dike. Borstein, Reichenbach, Odenwald. is a great difference in elevation between the upper stream course and its mouth, the neighboring rock may be cut into deep valleys and gorges. An extreme case is the grand canyon of the Colorado (Fig. 53) which has a depth of more than a thousand meters. Especially steep are the walls of water-cut channels when the materials of ROCK WEATHERING 93 which they are composed consist of porous deposits of clay or tuff, such as the porous tuff deposits of the Eifel (Fig. 54), or the loess. Streams with a lower gradient have broader valleys with great masses of talus at either side, and the larger blocks of this fallen material usually preserve their angular outlines, in contrast with the rounded bowlders formed by ice or flowing water. Rocks which are hard and able to withstand chemical weathering are least cor- FIG. 57. The devil's wall, a denuded basalt dike. Oschitz, Bohemia. roded, so that, after denudation, they stand out in great jagged peaks, like the dolo- mites of South Tyrol (Fig. 55). Even at lower altitudes, pegmatites and massive quartz dikes stand out prominently above the surrounding weathered material (Fig. 56), and serpentine forms projecting knobs. Locally, also, steep-walled dikes of basalt may stand out from more friable country-rock, an especially grotesque dike being shown in Fig. 57. Lamprophyres generally weather more easily, and therefore ordinarily appear as shallow trenches instead of as ridges. FIG. 58. Map of the south coast of the Isle of Arran. The trap dikes are promi- nently exposed by abrasion. The effects of abrasion are similar to those of erosion but naturally are confined to seacoasts. The breakers act upon the softer rocks first. If these are horizontal, they are undermined; if they run at an angle to the coast, they are cut into deep bays. An excellent example of the latter is shown on the south coast of the Isle of Arran (Fig. 58), where hundreds of trap dikes stand out like walls above the easily weathered sandstone, and extend far into the sea. VI. THE NATURE OF THE SEDIMENTS LITERATURE E. AND REE: "Die Diagenese der Sedimente, ihre Beziehungen zur Sedimentbildung und Sedimentpetrographie." GeoL Rundschau, II (1911), 61, 117. Idem: "Uber Sedimentbildung am Meeresboden. I. Teil." GeoL Rundschau, III (1912), 324. H. BOEKE: "Ubersicht der Mineralogie, Petrographie und Geologie der Kalisalzlager- statten." Berlin, 1910. G. BORNEMANN: "Uber den Buntsandstein in Deutschland." Jena, 1889. C. CLEMENT: "tiber die Bildung des Dolomits." Tscherm. min. petr. Mitt., XIV (1895), 526. G. R. CREDNER: "Die kristallinischen Gemengteile gewisser Schiefertone und Schiefer." Zeitschr. ges. Naturw., Halle, LXIV (1874). C. W. GUMBEL: "Die am Grunde des Meeres vorkommenden Manganknollen." Sitzb. bayr. Akad. Wissensch., 1878, 189. F. HOPPE-SEYLER: "Uber die Bildung von Dolomit." Zeitschr. deutsch. geol. Ges. t XXVII (1875), 520. G. KLEMM : " Mikroskopische Untersuchungen iiber psammitische Gesteine." Ibidem, XXXV (1882), 1. E. KOHLER: "Uber die sog. Steinsalzziige des Salzstocks von Berchtesgaden." Geogn. Jahresh., XVI (1903), 105. D. KREICHGAUER: "Die Aquatorfrage in der Geologie." Steyl, 1902. L. LEMIERE: "Transformation des vegetaux en combustibles fossiles." Compt. rend, VIII congr. geol. intern., 1900. Paris, 1901, 502. H. MONKE UND F. BEYSCHLAG: "Uber das Vorkommen des Erdols." Zeitschr. prakt. GeoL, XIII (1905), 1, 65, 421. J. MURRAY ET A. F. RENARD: "Les caracteres microscopiques des cendres volcaniques et des poussieres cosmiques et leur role dans les sediments de mer profonde." Bull. mus. roy. hist. nat. Belgique, III (1884), 1. Idem: "Notice sur la classification, le mode de formation et la distribution ge"o- graphique des sediments de mer profonde." Ibidem, 25. C. OCHSENIUS: "Die Bildung der Steinsalzlagerstatten und ihrer Mutterlaugensalze." Halle, 1877. Idem: "Die Bildung machtiger mariner Kalkabsatze." Neues Jahrb., 1890, II, 53. Idem: "Die Bildung von Kohleflozen." Zeitschr. deutsch. geol. Ges., XLIII (1891), 84. Idem: "Kohle und Petroleum." Zeitschr. prakt. GeoL, 1896, 65. J. H. VAN'T HOFP: "Uber die Auskristallisation komplexer Salzlosungen bei konstan- ter Temperatur unter Beriicksichtigung der natiirlichen Salzvorkommnisse." Zeitschr. angew. Chemie, XIV (1901), 531. J. H. VAN'T HOPP, W. MEYERHOPER, UND NORM. SMITH: "Untersuchungen liber die Bildungsverhaltnisse der ozeanischen Salzablagerungen, insbesondere des Stass- furter Salzlagers, XXIII. Abschluss und Zusammenfassung." Sitzb. preuss. Akad. Wiss., 1901, 1034. E. PHILIPPI: "tJber Dolomitbildung und chemische Abscheidung von Kalk in heu- tigen Meeren." Neues Jahrb., Festband, 1807-1907, 397. 94 THE NATURE OF THE SEDIMENTS 95 H. POTONIE: "Entstehung der Steinkohle und der Kaustobiolithe iiberhaupt." 5 Aufl., Berlin, 1910. E. RAM ANN: "Einteilung und Benennung der Schlammablagerungen." Monatsber. deutsch. geol Ges., 1906, 174. JOH. WALTHER: "Das Gesetz der Wustenbildung." Berlin, 1900. Composition of the Sediments. The materials forming the sediments are derived both from the weathering solutions and the weathered residues. If the weathered residues come under the influences of the transporting forces which act at the surface of the earth, they are carried away to form the various mechanical sediments, ceolian (Aiolos, God of the winds), alluvial (Lat. alluvius, ad, against, luere, to wash), or glacial (Lat. glacis, ice), depending upon their mode of transportation by wind, moving waters, or ice. True chemical sediments are formed from the weathering solutions by simple concentration, while organogenic sediments originate by the action of organisms upon such solutions. Since sediments are derived from the destruction of primary rocks, they are also called secondary rocks. There is a marked difference between the chemical compositions of igneous rocks and sediments, due to the separation of the former by chemical weathering into a residuum and a solution, and the subsequent mechanical separation of the residuum into deposits of different- sized grains. This difference is especially marked in rocks derived from solutions, but it may be clearly seen, also, in most seolian and alluvial sediments. In glacial deposits it is much less strongly developed, since here chemical weathering is of slight importance, and transportation, in general, produces no sorting of the material. Sediments may have approximately the composition of igneous rocks where the weathered material is deposited in slowly moving waters near the original locality, or where porous volcanic ejecta- menta are deposited by so-called mud-flows. Such sediments, however, are only of very local significance. In general, the dif- ference is very marked, and normal sediments of all groups differ decidedly in composition from normal igneous rocks. Mechanical Sediments. Mechanical sediments are derived from the weathered residues, therefore their compositions, on the whole, are alike. The transporting agents separate the material, primarily, according to size of grain, and upon this a classification of mechanical sediments may be based. The finest particles of the weathered residues, which hardly reach >{ mm., are called 96 FUNDAMENTAL PRINCIPLES OF PETROLOGY dust, or if wet, mud. They form the pelites (Gr. 71-77X6$, mud). Since they consist chiefly of the clay-like weathered products of the feldspars, they are also called argillites. The grains of coarser material or sand, may reach several millimeters in diameter, and when solidified are called psammites (Gr. ta^os, sand) or sandstones. Still coarser material is called gravel, and this grades into very coarse rubble. The few sediments composed predomi- nantly of these coarse materials are called psephites (Gr. \l/rj 85)j and are found within the igneous rock itself, or in the surrounding contact-metamorphosed zone, in the form of dikes, schlieren, or independent nodules. Where they occur within the parent igneous rock they are usually coarse- grained aggregates of the same constituents as those found in the main mass. They represent a continued growth of the individual POST-VOLCANIC PROCESSES 143 constituents, and the boundary between the two rocks is blended and indistinct. This, and the irregular shape of the intrusions, show that the pegmatites were formed during the later stages of the solidification of the main mass, and that, with the aplites, they represent the first extrusions after the original igneous activity. Pegmatites related to each type of plutonic rock are known. They are most commonly and most abundantly developed in connection with granites and nephelite-syenites, but are relatively rare with plagioclase rocks. The more basic the plutonic rock, the simpler are its satellites. The pegmatites normally have aplitic fades, and in places pass through all possible transitions to aplites. But lamprophyres also, here and there, have pegmatitic habits. Coarse-grained portions of kersantite dikes, and some pegmatite-like nephelinite schlieren in basalts, show undoubted affinities with true pegmatites. Pegmatites have coarse to very coarse textures, and may vary greatly both in texture and composition. Eutectic mixtures, causing parallel intergrowths of the individual minerals, are espe- cially widespread, for example in graphic granite or pegmatite in the narrow sense. The development of gigantic crystals and of crystal-druses suggest the origin of these rocks from magmas especially rich in mineralizers. This mode of origin seems a certainty after a close examination of the minerals themselves and of the intense solvent power which the melt had upon the con- stituents of the country-rock. In some cases this assimilation of foreign constituents was so great that it altered the entire character of the rock. A careful examination of the minerals of the pegmatites shows that there must have been a remarkable concentration of mineralizers, and that the so-called rare elements, which are present in normal rocks only in traces, are very abundant. The minerals of the pegmatites may be grouped as follows: 1. The minerals of the parent rock, namely: quartz, orthoclase, albite, and white and more rarely dark mica; and in certain rocks, anorthoclase, nephelite, sodalite, etc., with segirite and arfvedsonite. 2. Minerals containing especially active elements: tourmaline, topaz, fluorite, scapolite, and apatite. 3. Minerals containing rare elements : monazite, xenotime, orthite, beryl, chryso- beryl, niobates and tantalates, molybdenite, zircon, titanite, and, in nephelite-syenite pegmatites, the zirconium silicates, especially lavenite, mosandrite, rinkite, astrophyl- lite, katapleite, etc. 4. Minerals derived from the country-rocks: andalusite, disthene, garnet, cordier- ite, staurolite, and the like. 10 144 FUNDAMENTAL PRINCIPLES OF PETROLOGY To these must be added occasional ore-minerals, and, as final products showing the gradual transition into the thermal stage, various zeolites and some opal. This extraordinary paragenesis occurs only in the alkali-rich rocks. The peg- matites of the plagioclase series are much simpler, in many cases being merely coarsely developed phases of the parent rock. Crystal-druses are usually very abundant in granite-, syenite-, and nephelite-syenite-pegmatites, and there may be a remarkable development of beautifully crystallized minerals; in the plagioclase-pegmatites, druses are rarely present. Pegmatites rich in tourmaline, scapolite, etc., may impregnate the country-rock with these minerals to a considerable distance. Ordinarily, however, the minerals developed depend to a large extent upon the composition of the country-rock. For example, where the granite-pegmatites of the Bavarian Forest pass into the aluminium- rich contact-rocks, large druses containing andalusite are developed. In the Fichtel- gebirge a similar rock cuts eclogite and becomes a coarse-grained aggregate of feldspar and zoisite. In other regions the constituents dissolved from the country-rock form staurolite, garnet, disthene, etc. The orthoclase content of these dikes is readily lost and albite takes its place, in many cases becoming an important constituent. That there is a connection between the pegmatites and vulcanism has been denied on the strength of these abnormal relationships, and these rocks have been ascribed to lateral secretion, that is to the leaching of the country-rock by circulating water. The manner of their occurrence, however, is decidedly against this view, and their intimate connection with igneous rocks is too apparent. Furthermore, they do not bear the slightest resemblance in texture or composition to deposits from vadose solutions. Further modifications may be produced in the pegmatites by the action of piezo- crystallization. In many cases masses of alkali-mica scales take the place of the feld- spars, and normal pegmatite dikes, which extend for long distances, may grade into schistose aggregates of mica closely resembling mica-schists. The drusy texture is then usually entirely wanting. Such modified pegmatites, carrying staurolite and disthene derived from the contact-metamorphosed country-rock, occur in the Tessin (Ticino) paragonite-schists. On the other hand, perfectly normal pegmatites are found in the central granite. Like the mineral-rich dikes of the Titan-formation, which cut similar rocks, they contain innumerable druses filled with magnificent crystals. This drusy condition indicates a cessation of pressure during their formation ; in fact, the dikes intruded immediately after the main granite of the central Alps show in many places that orographic forces were absent. The Amygdaloids. Vesicular and scoriacious rocks, with their cavities entirely or partially filled with secondary minerals, are widely distributed among dike-rocks, especially among lampro- phyres. They also occur among the extrusives, and are as com- mon among basic rocks as among silicic. Such rocks (Fig. 86) are called amygdcdoids (Gr. d,uuy 5dXr7, almond) . While their charac- teristic paragenesis has usually been ascribed to lateral secretion, whereby the mineral-matter is supposed to have been deposited from solutions leached by atmospheric agents from the country- rocks, as a matter of fact these minerals are predominantly those which elsewhere are universally associated with igneous rocks, and which have never been known to be deposited from vadose waters. POST-VOLCANIC PROCESSES 145 Among the amygdule fillings, chalcedony and the zeolites are especially interesting. These minerals are elsewhere known only in connection with hot juvenile springs, and therefore probably have a similar mode of origin where they fill the vesicules. Most typical occurrences are agate-amygdules. These vary in size from a few millimeters to over a meter, and their banding (Fig. 87) clearly shows the gradual filling of the cavities. Various features suggest that the chalcedony was originally gelatinous and later became crystalline. In many cases the agate-amygdaloids are hollow and lined with quartz crystals, commonly amethyst, a form of quartz which is never of unquestionable vadose origin. Among other minerals occurring in this manner are calcite and aragonite, various zeolites, here especially well developed, prehnite, boron-bearing datolite, and, in certain melaphyres, abundant native copper. Scaly hematite and goethite are also present. Further, green substances of various kinds, such as seladonite or chloritic minerals, are abundant in the vesicules, and with them, in many cases, epidote or different colloidal silicates. These minerals are found not only in the vesicules of the lava-streams but also in the larger cavities of bombs lying in volcanic tuffs, and here and there in fissures. FIG. 86. Melaphyre amygdaloids. Oberstein a. N. FIG. 87. Agate showing conduit. Oberstein a. N. The vesicular character of the lava is doubtless primary and due to the escape of the gaseous mineralizers during the solidification of the rock. Besides these rounded vesicules, however, there are found in the effusive rocks many other cavities which are of irregular shapes and appear to be due to corrosion. These cavities probably origi- nated in the dissolving action of gases during the last stages of the solidification of the magma, and in them, minerals of an entirely different kind are deposited. Thus, tridymite and topaz are found in rhj^olites and trachytes, while well-developed crystals of sanidine, nephelite, sodalite, leucite, melilite, etc., as well as olivine, pyrox- ene, amphibole, and locally calcium-magnesium garnets, and even the rare lievrite, occur in the soda-rocks. Probably all of these minerals are of pneumatolytic origin, and water had little to do with their formation. The local occurrence of opal, however, shows that pneumatolysis was sometimes combined with the hydration of the thermal period. Finally, there remain to be mentioned certain peculiar, mostly colloidal phosphates which occur predominantly in fissures of volcanic rocks, and which unquestionably also belong to the post-volcanic thermal period. The turquois deposits in trachytes and trachyte tuffs of Persia and the amorphous phosphorite films on the parting- planes of many basaltic columns are of this kind. 146 FUNDAMENTAL PRINCIPLES OF PETROLOGY Mineral-dikes and Ore -veins. By far the greater number of fissure deposits of minerals and ores are of juvenile origin, and while certain ore deposits may be far removed from any igneous rocks, an examination of their general relationships very rarely shows them to be of vadose origin. Most mineral-dikes and ore-veins are of less importance in petrology than they are in mineralogy and economic geology, therefore only a few especially typical examples will be cited. Tin deposits are the most noteworthy and typical representatives of the pneu- matolytic period of volcanic activity, a period characterized especially by mobility of gases under high pressure. These gases, emanating from the magma and loaded with powerful mineralizers, penetrated fissures and dikes, and generally saturated the country-rocks to great distances. The ore deposits are genetically connected with Graphite lenses Gneiss FIG. 88. Occurrence of graphite lenses in the neighborhood of Passau. granitic rocks which either contain a very small amount of tin and other minerals of the dikes (tin-granite), or whose mica carries lithium (lepidolite-granite or its porphyritic equivalent, lithium-bearing quartz-porphyry). The nature of these ore-bearing dikes is well shown by the extent of the mineraliza- tion of the country-rocks, for whether the walls are granite, granite-porphyry, or quartz-porphyry, or injection-schists or other contact-rocks, they are impregnated to a considerable distance by quartz, tourmaline, topaz, fluorite, and other minerals of the tin formation. Most noteworthy is the fact that the feldspar of the original granite has disappeared, and topaz- and fluorite-rich greisen is formed. In the granite-porphyry called luxullianite, the groundmass is changed to a dark, tourmaline- quartz aggregate, the larger feldspars either remaining unaltered or becoming changed to tourmaline, topaz, or tin ore. Many rocks have been altered to tourmaline- or topaz-rich quartzites (zwitters), while the entire feldspar content of others has become kaolinized. All rocks, irrespective of their original character, show alterations of this kind, and everywhere near the dikes, cassiterite and its constant companion, arsenopyrite, occur as impregnations. Farther from the dike the normal, unaltered rock appears. Among ore-veins none furnishes a stronger argument against the lateral secre- POST-VOLCANIC PROCESSES 147 tion theory than does cassiterite. Here, contrary to the theory, the country-rock has in many cases been altered by the action of true fumaroles to a considerable dis- tance from the dike, and the ore formation was everywhere accompanied by the forma- tion of tourmaline and topaz, which were no more deposited from vadose circulating solutions than was the cassiterite itself. The phenomena connected with tourmaline- bearing copper-ores are analogous, and zwitter and greisen are developed, although to a less extent. The effects of post-volcanic processes are very characteristically shown by certain graphite deposits, especially by those occurring in dikes in the granulite of Ceylon, and by the graphite-gneiss which is found in small lenses in injection-schists near the granite-contact at Passau, Bavaria (Fig. 88). In the latter especially, the action of volcanic agencies is very clear. The parts rich in graphite represent impregnations of the injection-schists, for wherever graphite occurs they are more or less completely altered to kaolin, nontronite, and amorphous silicates of manganese, and in many cases are impregnated with opal. Injection-schists are ordinarily very compact rocks and form steep cliffs, but in the graphite area they are altered to unconsolidated earth. On the other hand, the younger lamprophyre and aplite dikes, which cut and fault the graphite lenses, have a a a a .a a a a Gabbro Scapolite-gabbro Apatite dikes FIG. 89. Apatite dikes with scapolitized gabbro. Husaas, Norway. (After] J. H. L. Vogt.) remained unaltered in spite of their high pyrite content. There can be no question as to the secondary character of the graphite in the injection-schists, nor of the intimate relation between the intense metamorphism and the formation of the graphite. The large amounts of iron and manganese which are present can only have been brought in with the graphite, for since these metals occur in their highest states of oxydation they cannot be ascribed to reducing agents. Probably most of the graphite was brought in as unstable carbon compounds, which easily broke up into carbon dioxide and oxides of the metals. At any rate, these graphite deposits are due to post- volcanic processes. The alterations in the country-rock around the rutile-apatite dikes of South Nor- way also point to intensive post-volcanic processes. The country-rock is a normal gabbro, but near the dikes it is impregnated with chlor-scapolite, and in many cases is entirely altered to a hornblende-scapolite rock filled with rutile and apatite (Fig.89). The central Alpine Titan formation is somewhat different in appearance, but it is very intimately related to the pegmatites of the central granite. Here the country- rocks had a great influence upon the dikes. This influence can be explained only on the assumption that hot gases had less to do in changing the character of the dikes than had heated solutions, which dissolved considerable material from the surround- ing rocks. The dikes are characterized by the constant presence of titanium oxide, 148 FUNDAMENTAL PRINCIPLES OF PETROLOGY although this does not occur in quantities great enough to be of commercial impor- tance. Where the dikes cut the granite, the chief constituents are quartz and adula- ria, with rutile, anatase, and brookite. In many cases these minerals form beautiful crystals, and the so-called Crystal Cellar of the Alps occurs in this part of the formation. Toward the contact-zone, prehnite, zeolites, and titanite, and farther on diopside, zoisite, and epidote occur in the dikes. Where they pass into amphibolite or green- stone-schist the latter three minerals become the chief constituents, the quartz disap- pears, and in place of adularia, beautiful crystallized albite is developed. In many places minerals characteristic of the pegmatite itself also appear, among them are monazite, beryllium-bearing euclase, tourmaline, some fluorite and apatite, ore- minerals of many kinds, and even native gold. The dikes become still richer in minerals where they traverse any of the numerous serpentine stocks of the region, Andesite Ore veins Propylite FIG. 90. Sketch map of the neighborhood of Schemnitz, Hungary. as was shown on page 133. The influence of the intrusives upon the country-rock, on the other hand, is at most very slight. Opposed to these processes, in a certain way, are the replacement processes in the dikes of the so-called propylitized gold-silver formation. Various igneous rocks, chiefly andesites, are impregnated with pyrite adjacent to the ore deposits. They are also otherwise altered to greater distances, yet never outside the zone of influence of the dike itself (Fig.. 90). This alteration process is called propylitization, and by it the original anhydrous minerals of the igneous rocks were altered to hydrous min- erals such as chlorite, sericite, kaolin, etc. Propylite (Gr. irpoiniXop, entrance, since the rocks were supposed to be the earliest extrusives of the Tertiary igneous cycle) was formerly supposed to be an independent kind of rock, intermediate between the older and younger igneous rocks. It actually is only an altered variety of andesite, and perfectly fresh rock occurs farther from the dikes. Propylitization is a form of pneumatohydatogenic alteration in which the country-rock had no influence upon the character of the dike. POST-VOLCANIC PROCESSES 149 Varieties of Rock Alteration. The replacement phenomena associated with these secondary formations are transitional to true post-volcanic metamorphism. The characteristics of the latter as a deep-seated process, and the differences between it and weathering have been mentioned previously. The local causes for replacement are very variable, as is also the ability of the different minerals to resist the different processes, consequently the course of the reaction itself and the results to which it leads are also variable. Minerals which can best withstand weathering are in some cases easily destroyed by replacement, and the contrary is just as commonly true. Tourmaline and disthene, for example, may be altered to mica-like substances by post-volcanic agents, but they are entirely unaffected by simple weathering. Again, biotite, monazite, and xenotime are absolutely fresh in completely kaolinized granites, yet they are very readily decomposed by weathering. The most important post-volcanic metamorphic processes are the following: 1. Kaolinization. Kaolinization is probably one of the most characteristic of replacement processes, and is of rather widespread occurrence. It is of most importance in granites and quartz- porphyries, but takes place in other rocks, and is even present in very basic varieties. The alteration normally appears in isolated, larger or smaller patches, or in a series of such patches along a fissure. It differs from weathering in that it primarily attacks the feldspar, acting upon plagioclase more readily than upon orthoclase, but scarcely affecting microcline. Weathering changes granite into rusty grush in which the alkali content remains high, but kaolinization completely removes the alkali as well as the lime from both plagioclase and orthoclase. Biotite is usually the first mineral to be affected by weathering, but it is fresh in many kaolinized rocks. It is especially characteristic that apatite, which is unaffected by chemical weathering, disappears completely under the action of kaolinization, and monazite and xenotime, which become cloudy upon the slightest weathering, are always clear and fresh in kaolin. The kaolinized patches in granite, even directly at the surface, laterally pass abruptly into normal rock. In depth, on the other hand, and this must be especially emphasized on account of innumerable assertions to the contrary, such a transition can nowhere be seen. It is true that drill-holes may show compact granite below the kaolin, but this is probably due to the irregular form of the kaolinized patches, as is shown in Fig. 91, in which the broken lines represent three drill-holes. Where the degree of kaolinization alters, it invariably increases with depth, even to 400 to. 500 meters, where atmospheric agents must of course be absent. Nests of tourmaline are not uncommon in kaolin deposits, and the purest kaolin occurs in their vicinity. 150 FUNDAMENTAL PRINCIPLES OF PETROLOGY Furthermore, mechanical rock analyses show the presence of a small quantity^of tourmaline, topaz, fluorite, pyrite, and siderite in numerous kaolin deposits, and these minerals, which do not occur in the original rocks, indicate the nature of the agency which produced the change. Granite Kaolin FIG. 91. Ideal section through a kaolin deposit in granite. Many kaolinized rocks are associated with deposits produced by pneumatolysis, a process which always produces intense alteration in the accompanying rocks. Thus propylite, and rocks within the sphere of influence of some graphite deposits or cassit- erite dikes, are altered to kaolin in the last stage of replacement. Furthermore, Faults KARLSBAD \ :[' Sprudel FIG. 92. Sketch map of the neighborhood of Carlsbad. Shows the relationship between the kaolinized patches and the fault lines. near many kaolin deposits, as at Karlsbad in Bohemia (Fig. 92), there are active hot springs even at the present time, and the relationship between the two is very clear. The fact that pegmatite and aplite are kaolinized in so many cases and so intensely shows the activity of the gases and vapors producing this type of alteration. POST-VOLCANIC PROCESSES 151 2. Saussuritization, uralitization, and the formation of green- stones, are alteration phenomena in basic igneous rocks, but, as was shown on page 131, they may also appear in granite itself as the result of contact-metamorphism. Alteration may take place, however, without the influence of foreign plutonic rocks, as was shown under the discussion of propylite, a rock belonging to the greenstone series. Under such metamorphism, the original tex- ture of the rock generally remains quite distinct. The feldspar, in some cases, is altered to a dense aggregate of saussurite, and such greenish, saussuritized porphyrites and other similar rocks are among the toughest and most resistant of rocks. Besides the calcium-aluminium silicates which compose the saussurite sericite and calcite may occur, and in some cases, as in normal greenstones, may predominate in amount. In such cases, the pyroxene is rarely truly uralitized, but is changed to an aggregate consisting chiefly of chlorite with some epidote and perhaps a little uralite. Primarily the process is one of hydration and decalcification. An impregnation by pyrite is always associated with the formation of greenstone, and pyrite is probably always secondary in saussuritized and uralitized rocks. Greenstones are most commonly formed from silica-poor roeks. They are rarely formed from those that are rich in silica, and apparently never from sodic rocks. Dikes and extrusives are most subject to this alteration; among the former, lampro- phyres, and among the latter, andesite and porphyrite, trap, and melaphyre are espe- cially susceptible. Two entirely different types of alteration may be distinguished among these rocks. In the first type, the rocks, which are gray or brownish black when fresh, become brown, brownish red, or red. These are the usual colors of altered quartz-porphyries, and they are not uncommon among basic porphyrites and melaphyres. The origin- ally compact rock eventually acquires an uneven fracture and loses its luster, and its marked clay-like odor has given rise to the name clay-stone porphyry. Finally, the rocks take on a dull uniform color, and form masses which are distinguishable with difficulty from each other. They were called wacke by the earlier geologists. The final phase of this alteration process makes the rock character unrecognizable, and is doubtless due to atmospheric agents. Whether the beginning of the alteration, which is marked by an intense impregnation with iron oxide, is due to the same cause, must still be considered questionable. The alteration shown by greenstone-porphyry and diabase is quite different. Here the entire process was doubtless one of true replacement by means of hot juvenile waters, which were probably always sulphur bearing, as seems to be indicated by the constant presence of pyrite in rocks so altered. The formation of saussurite has often been considered typical of dynamometamor- phism on account of the relatively high specific gravity of the calcium-aluminium silicates which are present in it. As was shown above, most saussuritized rocks result from contact-metamorphism. The alteration is not necessarily connected with 152 FUNDAMENTAL PRINCIPLES OF PETROLOGY especially intense tectonic processes, but depends upon causes similar to those produc- ing greenstones, the difference being due perhaps to greater depth and pressure during the process of replacement. More hypabyssal and plutonic rocks than extrusives, therefore, are saussuritized, while there are more greenstones derived from extrusives. 3. Sericitization in acid rocks corresponds somewhat to the formation of greenstone from those that are basic. Sericitized aggregates derived from feldspars occur even in the latter, while chlorite is widely distributed in sericitized granites or quartz- porphyries. Sericite occurs locally in the Erzgebirge at the con- tact between granites, gneisses, and quartz-porphyries and veins of silver-bearing galena, the formation of the latter having pro- duced the alteration in the 'former. It also occurs where rocks show considerable faulting, for example in the Bavarian Forest where the granite is altered to the Pfahl schist, a rock consisting of finely brecciated quartz separated by fine sericite films. The micaceous character of the alteration is especially dis- tinct in the fine, silky-lustered, light-colored sericite-schists pro- duced from crushed quartz-porphyries. In these rocks, which in many cases have paper-thin schistosity, the rounded or embayed quartz phenocrysts of the former quartz-porphyry may still be recognized, and they may even be visible megascopically as small, hard knots on cleavage surfaces of the crinkled rock. Sericitization also has frequently been considered a dynamometamorphic change, and it is undoubtedly true that, in numerous cases, mechanical forces have assisted in the alteration. Probably the most characteristic examples are offered by the Pfahl schists and certain poryhyroids occurring in the strongly folded Paleozoic rocks of the Taunus and the Ardennes, the latter having been altered to sericite-schists where mashed to an exceptional degree. That thermal activity was here of great importance is shown by the enormous quartz-dikes (and their accompanying minerals) which are associated with the Pfahl schists and with other closely related alteration products of the crushed granite. In other cases, however, Sericitization has altered quartz- porphyry to a silky-lustered, white schist without producing any observable deforma- tion in the quartz phenocrysts. Many such schists are interbedded with contact- rocks, and it is probable that the Sericitization of the quartz-porphyry is here due to contact-metamorphism. It is hardly necessary to say that quartz-porphyry-tuffs under analogous conditions are similarly altered, so that after metamorphism the igneous rock cannot be separated from its tuff. 4. Serpentinization. The formation of serpentine from olivine- rocks may be traced in most cases to the action of contact-meta- morphism, as described on page 132. Some serpentines, however, may have originated from originally anhydrous olivine-rocks by thermal processes following their intrusion, just as certain green- stones originated by a similar process. Nevertheless, large POST-VOLCANIC PROCESSES 153 mountain-making masses of serpentine are certainly not products of normal weathering, and just as little are they the results of dynamometamorphism. The attempt has been made to assign different modes of origin to the two varieties of serpentine; antig- orite, which occurs principally in folded mountain regions, is regarded by some as the result of the erogenic forces, while chrysotile is assumed to be the normal product of weathering. Serpentine [(Mg,Fe)O : SiO 2 = 3:2] may be regarded as consisting of one part olivine [(Mg,Fe)O : SiO 2 = 2:1] and one part bronzite [(Mg,Fe)O : SiO 2 = 1:1], but it is doubtful if olivine and bronzite in these proportions ever resulted in the formation of serpentine. Most serpentines originated from peridotites, that is, from rocks in which olivine greatly pre- dominated, and this fact alone shows that the formation of serpentine is a more com- plicated process than is usually supposed since a considerable amount of silica must be added or great amounts of magnesia and iron subtracted. Furthermore, the develop- ment of other new minerals, such as talc, actinolite, chlorite, and carbonates, indi- cates that the process is not simple. With the exception of a few unimpor- tant contact-metamorphosed carbonate- rocks, all serpentines were derived from rocks originally igneous. In innumerable cases they accompany greenstones and greenstone-schists, which are undoubtedly & . . , , FIG. 93. Serpentimzed olivine of igneous origin, and they are widespread cr y sta ls in picrite. Trogen, near Hof , as dikes. It is true that serpentines gener- Fichtelgebirge. ally k form lens-shaped beds intercalated be- tween other rocks, but where they are interbedded with limestones and marls, they are surrounded in many cases by normal contact-rocks. Like the accompanying greenstone and saussuritefels, serpentine is probably never an original rock. Although in some cases it has been considered original, microscopic examinations almost invariably show, by the more or less abundant remnants of the anhydrous silicate still present, or by the mesh texture of the chrysotile or the grating texture of the antigorite, that olivine was the chief original constituent. Where olivine occurred as a subordinate constituent in such rocks as gabbros, traps, melaphyres, and basalts which have been serpentinized, its outlines are still well preserved. In true serpentine masses such well-bounded pseudomorphs are wanting, the original rock having consisted originally of an equigranular aggregate of olivine. Porphyritic olivine-rocks are extremely rare, but where, they do occur, the phenocrysts with their mesh structure and the preserved remnants of the original mineral, stand out distinctly from an extremely 'dense serpentine groundmass (Fig. 93). 5. The formation of talc must be considered here as an appendix to serpentinization, since talc may be produced by this process as 154 FUNDAMENTAL PRINCIPLES OF PETROLOGY an accessory mineral. Talc differs from serpentine, which is de- rived almost exclusively from olivine, in being an alteration prod- uct of many minerals, magnesia-bearing and magnesia-free, silicate and non-silicate. It may be formed from olivine-rocks as well as from dolomites, from limestones as well as from granites. Even true slates may be re-formed into pure talc-rocks. This most rad- ical replacement process uniformly attacks everything that comes within reach of the re-forming magnesia-rich solutions, and is rather widespread in the contact- zones of granites, where such solutions are always strikingly abundant. Important deposits of compact talc which were formed in this manner in all probability originally consisted of the colloidal alteration products of various minerals and rocks at the granite contact. Where talc-rocks occur as patches and dikes in serpentine, they are more distinctly crystalline than they are elsewhere. In many cases much chlorite is developed simultaneously with the talc, as in soap- stone, which occurs in granite as well as in serpentine. Furthermore, many of the granites in the neighborhood of the talc beds of the Fichtelgebirge are intensely chloritized, and pseudomorphs of chlorite after orthoclase are not uncommon. Fi- nally, in completely mashed granites, such as the so-called Winzer granite on the banks of the Danube east of Regensburg, the entire feldspar content is in many cases altered to a dense aggregate of chlorite. 6. Zeolitization is a replacement process which primarily attacks nephelite, leucite, and minerals of the sodalite group. That this alteration is also due to thermal action is shown by the fact that zeolitized and perfectly fresh rocks occur side by side in places where the hydrographic conditions are the same. The phonolite domes of Hohentwiel in Hegau, which are especially well known because they carry natrolite, are penetrated by a well to a depth of 100 meters, and all of the rocks taken from it are uniformly zeolitized, yet very small ejected fragments of the same rock in the surrounding tuffs contain no zeolites, even at the surface, but instead carry considerable amounts of perfectly fresh hauynite and noselite. 7. Besides these ordinary phenomena of post-volcanic metamorphism, others of purely local character should be mentioned, such as the alteration of rhyolites and rhyolite tuffs to alunite, and the formation of potash-rich seladonite in potash-free melaphyres. In both cases there has been a great addition of material such as could not have been brought about by the usual agents of weathering. The silicification of rhyolites and quartz-porphyries or their tuffs, and the develop- ment of chalcedony and opal in all kinds of igneous rocks, their tuffs, and the adjacent sediments, belong to the same group of post-volcanic processes. Silicification by weathering, on the other hand, generally produces a quartz-cement, yet such cement POST-VOLCANIC PROCESSES 155 may be also abundantly produced by post-volcanic processes. This may be seen, for example, at numerous contacts between diabase and silicified adinoles and lydites. The formation of bauxite is of a different character, for here great amounts of silica are removed, an alteration also observed in some basic igneous rocks. 8. Metasomatic Replacement of Carbonate-rocks. The constituents of carbonate- rocks, especially of limestones, are readily soluble in various solutions, and from these solutions new minerals crystallize. Thus ores or minerals which have originated through replacement are very widely distributed in limestones, and are spoken of Siderite Marble FIG. 94. Irregular form of a siderite patch in marble. Huttenberg, Karnten. as metasomatic (Gr. /zerd, after, o-w/ia, body). These deposits are extremely irregular in shape (Fig. 94), and may be rather sharply separated from the unaltered limestones or may be connected with them by transition zones. Numerous deposits of siderite, rhodochrosite, and magnesite, which are very abun- dant in the altered limestones of the contact-zone of the central Alpine granite, belong to this class. The analogous silicate skarn deposits with magnetite, manganese silicates and oxides, or zinc oxide, etc., were already mentioned on page 129. Finally, sphalerite, galena, and calamine may occur under similar conditions, as do also the silica deposits in greenstone-schists and amphibolites. IX. REGIONAL METAMORPHISM LITERATURE J. ROTH: "tlber die Lehre vom Regionalmetamorphismus und die Enstehung der kristallinischen Schiefer." Abhandl. preuss. Akad. Wiss., 1871, 151. Summar- izes the older literature. Idem: "fitude sur les schistes cristallins." IV congr. geol. intern., Londres, 1888. F.A. ADAMS AND J. T. NICOLSON: "An Experimental Investigation into the Flow of Marble." Phil. Trans. Roy. Soc., London, CVC (1901), 363. A. BALTZER: "Der mechanische Kontakt von Gneis und Kalk im Berner Oberland." Beitr. geol. Karte Schweiz, XX (1880). F. BECKE: "Beziehungen zwischen Dynamometamorphose und Molekularvolumen." Neues Jahrb., 1896, II, 182. Idem: "tlber Mineralbestand und Struktur der kristallinischen Schiefer." Denkschr. Akad. Wiss. Wein, CXXV (1903). H.CREDNER: "Uber nordamerikanische Schieferporphyroide." Neues Jahrb. , 1870, 970. Idem: "tTber die Genesis der archaischen Gneisformation." Zeitschr. deutsch. geol. Ges., XLII (1890), 602. U. GRUBENMANN: "Die kristallinischen Schiefer." I, 2 Aufl., Berlin, 1910. C. W. GUMBEL: " Geognostische Beschreibung des ostbayrischen Grenzgebirges." Gotha, 1868. A. HEIM: "Untersuchungen iiber den Metamorphismus der Gebirgsbildung." Basel, 1878. E. KALKOWSKY: " tlber die Erforschung der archaischen Formationen." Neues Jahrb., 1880, I, 1. J. LEHMANN: "Die Entstehung des altkristallinen Schiefergebirges." Bonn, 1884. R. LEPSIUS: "Geologic von Attika," Berlin, 1893. W. LOSSEN: "Geognostische Beschreibung der linksrheinischen Fortsetzung des Taunus." Zeitschr. deutsch. geol. Ges., XIX (1867), 509. L. MILCH: "Beitrage zur Kenntnis des Verrucano." Leipzig, 1892 and 1896. Idem: "Die heutigen Ansichten iiber Wesen und Entstehung der kristallinen Schie- fer." Geol. Rundschau, I (1910), Hf. 3. F. PFAFF: "Mechanismus der Gebirgsbildung." Heidelberg, 1880. H. H. REUSCH: " Die fossilfiihrenden kristallinischen Schiefer von Bergen." Leipzig, 1883. H. ROSENBUSCH: "Zur Auffassung des Grundgebirges." Neues Jahrb., 1889, II, 81. Idem: "Zur Auffassung der chemischen Natur des Grundgebirges." Tscherm. min. petr. Mitt., XII (1891), 49. A. SAUER: "Das alte Grundgebirge Deutschlands." Comptes rendus IX congr., geol. intern., 1903, Wein, 1904, 587. J. J. SEDERHOLM: "tlber den gegenwartigen Stand unserer Kenntnisse der Kristallin- ischen Schiefer von Finnland." Comptes rendus IX congr. geol. intern., 1903, Wein 1904, 609. W. SPRING: "Recherches sur les propriete"s que possedent les corps solides de se souder par 1'action de la pression." Bull. acad. roy. Belgique, 1880, 323. 156 REGIONAL METAMORPHISM 157 P. Termier: "Les schistes cristallins des Alpes occidentales." Comptes rendus IX congr. geol. intern., 1903, Wein, 1904, 571. E. WEINSCHENK : " M6moire sur le dynamo-me"tamorphisme et la piezocristallisation." Comptes rendus VIII congr. geol. intern., 1900, Paris, 1901, 326. Idem: "Beitrage zur Petrographie der Zentralalpen, speziell des Grossvenediger- stockes. III. Die kootaktmetamorphe Schieferhiille." Abhandl. bayr. Akad. Wiss., II Kl., XXII (1903), II Abt., 263. Idem: "tlber Mineralbestand und Struktur der kristallinischen Schiefer." Ibidem, XXII (1906), III Abt., 727. Early Ideas Regarding the Crystalline Schists. Geologists long ago recognized the existence of a fundamental difference between the so-called fossiliferous rocks and certain unfossiliferous crystalline schists which combine the characters of bedded and crystalline rocks. Many of the latter were definitely known to be of great geologic age, therefore, on the basis of their petrographic characters, the generalization was made that all schistose crystal- line rocks must belong to the same primitive Archean formation, a formation supposed to consist of the oldest sediments of the earth. With further study in the provinces which were best known at that time, this " primary" schist group was subdivided into gneiss, mica schist, and phyllite. Each of these rocks was supposed to be as characteristic of a time-division in the history of the earth as any of the later fossiliferous formations. Since these old deposits were unfossiliferous, their separation is necessarily purely petro- graphic and depends primarily upon the fact that the gneisses, which are the very oldest formations, are chemically allied to granitic rocks, while the other two groups are analogous to later clastic sediments. Furthermore, there appears to be a gradual decrease in the crystalline character of the rocks from the gneisses to the phyllites, and most of the upper members of the latter group appear to be transitional to clastic rocks. The assumption that the- Archean represents the original crust of the earth and the first chemical precipitates deposited upon it, appears at first sight to be the simp- lest and most natural explanation, and the fact that the crystalline schists were uni- versally present wherever deep cuts exposed the base of the fossiliferous formations, was formerly especially emphasized. Furthermore, it was generally found that the different members of the crystalline schists were the same everywhere, and they occurred in the same sequence gneiss, mica-schist, and phyllite as they neces- sarily should if they represented the oldest formations of the earth's crust. The innumerable inconsistencies which careful study reveals between the actual relationships and these theories might easily be overlooked on a superficial investiga- tion of the crystalline schists. Because some crystalline schists are unquestionably pre-Cambrian, all have been assigned to this age, yet in by far the most localities 158 FUNDAMENTAL PRINCIPLES OF PETROLOGY they are overlaid, not by the oldest fossiliferous sediments, but by very much younger ones, into which the schists in many cases pass by gradual transitions. On the other hand, the true sediments of the Cambrian, and those of the pre-Cambrian carrying occasional fossils, are by no means the oldest, non-crystalline formations of the earth, for in South Africa there is a whole series of non-fossiliferous sediments whose clastic characteristics are hardly altered, yet in it may be recognized a long period of pre-Cambrian sedimentation, divided into epochs by numerous well-developed unconformities and transgressions. Younger Crystalline Schists. Because most crystalline schists contain no fossils, they were thought to have been formed before organic life was possible upon the earth, hence the term Azoic (Gr. , without, coi>, life) was applied. Even so long ago as the beginning of the last century, occasional fossils had been found in rocks with the petrographic character of normal crystalline schists. This fact, however, did not disturb the belief in the non- fossiliferous character of the old crystalline schist formation, for the fossils were not 'especially primitive but could be correlated with certainty with type-fossils of younger geologic epochs. Thus the Silurian graptolites in the mica-schists of the Bergen peninsula in Norway, the Jurassic belemnites in those of the St. Gotthard region, and the Carboniferous plant remains in the phyllites of the Low Tauern mountains, are all similar to the type-fossils and show that these schists unquestionably belong to post-Archean formations. Elsewhere, gneisses and other crystalline schists are found interbedded with younger formations, for example in the Cretaceous in Attica. These " younger" crystalline rocks, consequently, were differ- entiated from the so-called Archean schists by their fossils, and enormous faults were artificially established, especially in the Alps, to separate the "true" crystalline schists from these later imitators. In petrographic character these younger crystalline schists correspond so completely in every particular with those regarded as Archean, that from this standpoint alone they must be considered identical. By so regarding them, however, the whole basis for classifying the " crystalline schists" as a single formation, namely on their petrographic character, is destroyed. Here, as everywhere, the fundamental law of petrography holds: petro- graphic character and geologic age are in no way related. The subject cannot be dismissed without mentioning the fact that there have been found locally in the crystalline schists, certain structures which seem to represent or- ganic remains, and which differ from all known fossils. For a long time these were regarded as examples of the earliest organisms which had existed upon the earth. REGIONAL METAMORPHISM 159 One of these apparent fossils is Eozoon Canadense (Fig. 95). This consists of serpen- tine and calcite so intergrown that it externally resembles certain organic structures and therefore was long thought to be a variety of primeval, giant foraminifera, coral, or something of that nature. Careful examinations of material from the original locality, Petite Nation, Canada, and of all similar ophicalcites from Europe, have clearly shown, however, that they are all contact-metamorphosed, forsterite-bearing limestones and not of organic origin. FIG. 95. Eozoon Canadense. Ophicalcite from Petite Nation, Canada. During the long controversy regarding the organic nature of these rocks, a note- worthy microscopic feature of the Canadian occurrence was overlooked, although just this feature simulated the organic structure, namely, a worm-like intergrowth of do- lomite and calcite (Fig. 96), somewhat resembling pegmatitic intergrowths of quartz and feldspar. Like these, it probably represents a eutectic mixture. Futhermore, absolute proof of the great age of the Laurentian deposits in which the Eozoon Canad- ense was found does not appear to have been given, the age assumption resting upon the petrographic character of the rocks. It is noteworthy that all the known fossils of the crystalline schists, so far as their ages have been determined with anything like certainty, belong to relatively young geologic formations. An examination of these so- called oldest sediments in various localities leads to the conclusion that they are un- questionably not the oldest fossil-bearing sediments. The same conclusion is reached by a study of the life development in the Cambiran, its well-developed fauna showing that it must have been preceded by a long series of more primitive forms. Nowhere in the crystalline schists, however, have fossils bearing the characteristics of primeval forms been found. This has been explained as being due to the lack of resistant hard parts in these primitive forms, an explanation also given for the fossil-free character of the extensive pre-Cambrian formations of South Africa. If, however, conclusions derived from historical geology are justifiable, there can be no doubt that Cambrian fossils must have had a long series of ancestors with pre- servable skeletons. That they have not been found has always been a stumbling- 11 FIG. 96. Eutectic intergrowth of calcite and dolomite in Eozoon. Petite Nation, Canada. 160 FUNDAMENTAL PRINCIPLES OF PETROLOGY block to geologists, and it seems impossible to explain their disappearance except on the assumption of great disturbances of the earth's crust at the beginning of the Cambrian, perhaps corresponding to the Catastrophic Period of Stiibel. Variability of the Crystalline Schists. The great variability in the composition of the different members of the crystalline schists series has already been pointed out. It is so pronounced that it can be overlooked only on the most superficial examination. It is true that the rocks of one formation are embraced under the name of gneiss, but the name is more geologic than petrographic, and if the rocks are examined carefully, all ideas of even approxi- mately similar characters must be dismissed at once. The individual members of the gneiss series have only two properties in common: (1) they have a more or less clearly recog- nizable parallel structure, and (2) they usually contain the minerals quartz, feldspar, and mica. The gneisses of the Alps, the Bavarian Forest, the Erzgebirge of Saxony, etc. are so very different in texture, in the occurrence of accessory minerals, and in the relative proportions of the chief constituents, that geologists accustomed to work in one region are hardly willing to regard the rocks in another as equivalent formations. Furthermore, innumerable rocks, granular limestones, gabbros, eclogites, amphibolites, serpentines, or ores, are interbedded with the gneiss, either in schistose or lens-like masses, or forming an entire horizon, as granulite does in the Saxon Granulitgebirge, in Bohemia, and elsewhere. In short, the whole appearance of the formation differs from that which one naturally would have expected of the first solid crust of the earth. The conditions in the mica-schist formation are still more confused. Besides the mica-schists proper, all of the characteristic rocks of the gneiss formation, amphibolites, chlorite-schists, green- stone-schists, calcareous-mica-schists, quartzites, etc., again appear. The same variability, though to a lesser degree, is characteristic for the phyllites, the upper group of the crystalline schist formation. A thorough geologic and petrographic study of the crystalline schists shows, without question, that these rocks by no means represent a universal formation. The only characteristic common to all of them is their variability and not their uniformity, although the latter might be presupposed as an essential of the earliest crust of the earth. It follows, therefore, that the crystalline schists cannot be portions of this primeval crust. REGIONAL METAMORPHISM 161 GiimbePs Theory of Diagenesis. Gumbel attempted in a measure to overcome the difficulties brought out by the characters of these rocks by means of his theory of diagenesis (Gr. Sid ,through, yljvofjiai, to be born). According to this theory, the crystalline schists, which were originally normal, clastic sediments, have undergone much greater alteration than the sediments of later geologic formations. This is due primarily to the fact that the water of the earliest periods, on account of its higher temperature and greater content of chemically active agents, had a much greater power to dissolve and rebuild than it had later. The recrystallization of the ooze, thoroughly saturated as it was with hot sea water, to gneiss or mica-schist is hard to understand, especially when the unimportance of diagenesis in later formations is considered. And furthermore, only the insoluble residues were left to be re-formed by diagenesis, the primeval ocean having undoubtedly leached and precipitated elsewhere some of the materials from the particles of ooze still floating in it. The theory of diagenesis depends upon the assumptions that all of the formations embraced in the Archean group represent one definite time period, that they were deposited before the oldest fossiliferous strata, and that during this time the physical condi- tions at the surface of the earth were not as yet suitable for life, or, if they were so, only for very low forms, These assumptions are accepted as a matter of course by most geologists, although the existence' of crystalline schists of definitely-known younger age alone should serve as a warning that care must be taken in using petrographic characteristics as indicative of definite age relationships. The crystalline schists of the Pyrenees which are overlaid by Upper Silurian beds, and the formations of the Central Alps overlaid by Triassic and Jurassic, cannot be called pre-Cambrian simply because they correspond petrographically with rocks which are known to be pre-Cambrian. It is hardly conceivable that normal Cambrian or Silurian sediments could be deposited in one place on the earth's surface while at the same time crystalline rocks were directly formed in another. The conditions necessary for the development of primary crystalline schists are so different from those of normal sedimentation, that the simultaneous formation of the two seems impossible. One must assume, therefore, as geologists generally do, that the two groups are of different ages, and all the crystal- line schists are Archean or pre-Cambrian; or that the crystalline schists were originally true sediments which developed a crystalline texture subsequent to their deposition, by some process independent of sedimentation. The "younger" crystalline schists definitely show that such metamorphism may take place, and their similarity to true Archean rocks suggested the possibility that these older crystalline schists 162 FUNDAMENTAL PRINCIPLES OF PETROLOGY were produced by some widespread alteration process. In this way the theory of general or regional metamorphism originated. Theories of Regional Metamorphism. Regional metamorphism, as opposed to contact-metamorphism which is distinctly a local phenomenon, has been regarded as due to alteration processes which have a very extensive zone of action. All theories of regional metamorphism proceed from the conviction that the crystalline schists generally do not possess the characteristics of primary formations. If, therefore, these rocks did not originate from a primitive crust and the oldest chemical sediments, they undoubtedly correspond to sediments and igneous rocks which FIG. 97. Injection schist. Freiberg, Saxony. occur in an unaltered condition on other parts of the earth's sur- face. The chemical composition of these altered rocks indicates the character of the original material ; simple recrystallization does not produce great chemical alterations, and there is a character- istic chemical difference between clastic and igneous rocks, as was shown in Sections V and VI. The crystalline schists have been classified, for example by Rosenbusch, on the basis of their chemical compositions. To such a classification the objection has been made that, owing to the great variation in the character of sediments, an arkose or greywacke may somewhere occur which corresponds closely to granite in chemical composition. While this is possible it is very improbable, and the chances are very slight that a gneiss of granitic composition was originally such a greywacke. On the other hand, the igneous rocks may have undergone extensive alteration before metamorphism, and thus a further disturbing factor is introduced. Such alteration REGIONAL METAMORPHISM 163 doubtless occurred in many cases, but the results of normal weathering would have slight influence on the enormous rock-complexes from which the crystalline schists must have been formed. With rare exceptions, the chemical compositions of the re- placement products, which were discussed in Section VIII, differ just as much from the chemical composition of the sediments, as do those of the original igneous rock itself. The chemical composition of a crystalline schist is always an important aid in the determination of the character of the original rock, and in certain cases it alone is sufficient. In other cases it may be necessary to confirm the determination by means of the geologic mode of occurrence. If the gneiss formation, for example, is exam- ined by these methods, it will be found that two end-members are easily distinguished. The lower part of the series, as a rule, shows the characteristics of true granites, quartz-diorites, etc., while the upper corresponds to greywackes, slates, and the like. FIG. 98. Granite made schistose by resorption. Mulda near Freiberg, Saxony. The latter, also, is distinguished from the former by its much more abrupt variation in habit and composition. According to their mode of origin, these rocks are called igneous gneisses or sedi- mentary gneisses, or, following Rosenbusch, orthogneisses or paragneisses. Between the two, however, there is a third equally justifiable group which possesses the chem- ical characteristics of neither igneous nor sedimentary rocks. Rosenbusch classified them as metagneisses. Certain of these rocks usually show rather perfect schistosity and banding and, like the paragneisses, vary abruptly in composition. These rocks are in part schists injected with granitic magma (Fig. 97), in which the phenomenon of injection can be still distinctly recognized megascopically and microscopically, in part mixed rocks in which the exfoliated schist was more or less completely resorbed by the granitic magma (Fig. 98). In many cases the conclusions as to the original character of a crystalline schist, drawn from its chemical analysis, may be confirmed by preserved remnants of the original texture. Thus, large quartz boulders, almost unaltered, may be preserved in metamorphosed rocks, or fragments of fossils may be found. In such cases there can be no question as to the sedimentary origin of the rock. On the other hand, the gneisses 164 FUNDAMENTAL PRINCIPLES OF PETROLOGY of the central Alps show, besides the usual granitic texture, all the other external characters of intrusive rocks, such as inclusions of the country-rock, apophyses, etc. Within them, also, dikes of chlorite-schist occur, and they may show more or less sharply defined light spots with the outlines of the phenocrysts of the original porphyrite (Cf. Fig. 83). Further, in the uralitic hornblende of the amphibolites there may still be distinct traces of the parallel diallage inclusions, which are so characteristic of gabbro. Thus similar examples could be presented for almost all varieties of crystalline schists, definitely proving that these so-called primitive rocks embraced both igneous rocks and sediments. An objection, which applies equally well to all theories of regional metamorphism, is that even in the very oldest sediments, namely, the basal conglomerates of the oldest fossiliferous formations, there have been found occasional boulders petrographically resembling crystalline schists. According to the theories of metamorphism, if rounded fragments of mica-schist and phyllites occur in the overlying Cambrian strata, these boulders must have been metamorphosed before the Cambrian rocks were deposited. In other words, the metamorphism must have taken place after the deposition of the upper beds of the phyllite and before the deposition of the lower beds of the Cambrian, in spite of the fact that the two series are connected in many cases by all possible transitions. Such occurrences will be discussed in greater detail at the conclusion of this section. Three theories of regional metamorphism are of especial importance:' 1. Plutonic (Pluto, God of the underworld) or anogenic (a va, up, ylyvofjiai, to be born) metamorphism. 2. Hydrochemical (Gr. #5 up, water), Neptunian (Neptune, God of the sea), or katogenic (Gr. Ks Gneiss Granite Mica-schist Pfal FIG. 105. The gneiss region of the inner Bavarian Forest. 178 FUNDAMENTAL PRINCIPLES OF PETROLOGY cynian gneiss formation of the Bavarian Forest. Taking into consideration the dip of the strata, the thickness is about 12 to 15 km. A cross-section through the region, for example near Zweisel, shows at least ten large and innumerable smaller areas of true haphazard granite, in all certainly amounting to more than 5 km. These 5 km. must therefore be subtracted from the total thickness of the complex. Further, the large granite masses show that a great granite massif is present beneath the whole region, the insignificant portions laid bare by erosion being simply apophyses from it. The granite not only occurs in these larger masses but it is injected into the schists everywhere in the region. This shows that the 15-km.-wide altered zone is only seem- ingly the extent of the metamorphism, the sedimentary rocks themselves actually being in much more intimate contact with the granite than is apparent at the surface. Toward the north, near Eisenstein, the granite impregnation ends, and only isolated patches of granite are found. The rock here is mica-schist, while still farther distant it is phyllite. As the distance from the igneous rock increases, the folding and crum- pling of the schist decreases, the so-called phyllites showing only very fine crinkling. If the rocks which have been altered by contact-metamorphism, and this includes most of the gneiss, be subtracted from the enormously thick crystalline schist-formation, there will remain but a small portion of the rocks to be included in the estimate of thickness. It must be remembered, also, that where narrow, uniform dikelets of igneous rocks intersect a region, they can only be apophyses of larger masses lying underneath. With these facts in view, the distinction between contact- and regional-meta- morphism, which always has been emphasized in geologic litera- ture, loses much of its significance, and it becomes still less im- portant when it is noted that the most characteristic rocks of con- tact-zones, such as "knoten-," "garben-," and chiastolite-schists, also occur in the crystalline schist area. The identical textures and mineralogic compositions of innumerable crystalline schists and contact-rocks show the analogy between the alteration processes by which they were formed. If all the gneisses which have been shown by, modern petrographic methods to be of igneous origin, and all those rocks which can be recognized as true injection schists, and finally all rocks which are demonstrably due to contact-metamorphism were separated from the crystalline schist formation, it would be much easier to grasp the meaning of the chemico-geologic processes which produced them than now when all these different varieties of rocks are dragged along as useless ballast. Viewed from the standpoint of StiibeFs theory of vulcanism, the following conclu- sions are reached. The original crust and the oldest sediments can never be examined, for they were repeatedly broken up by volcanic action, covered by igneous material, were resorbed, or lie buried at such depths that the deepest borings or mines cannot REGIONAL METAMORPHISM 179 reach them. An examination of the sediments and the fossils from the Cambrian to the present shows that the crystalline schists are not especially old. Variations in climate had already been established in the oldest geologic periods, and the interior heat had, at most, only a very slight effect upon conditions on the surface; that is, the crust at that time had already attained a considerable thickness. Long periods with conditions favorable for the existence of organisms preceded the Cambrian, at which time the climatic conditions upon the earth were nearly the same as those which exist at the present time. The struggle between igneous and sedimentary formations had been going on for many long geologic periods before the deposition of the oldest sedimentaries now known. The deep deposits which had been laid down upon the first solid crust of the earth had been completely broken up, and impregnated and altered by the igneous rocks, and from their fragments new beds were formed. The enormous igneous intrusions of the past probably altered the sediments very much more extensively, but in a manner analogous to, that in which the later rocks were altered. There are thus found, in the oldest clastic deposits, fragments of much older formations which have the characteristics of the crystalline schists. No certain proof exists that the crystalline schist boulders which occur in sedimen- tary rocks were derived from directly underlying crystalline schists, nor that they were already crystalline at the time of the formation of the sediments. For example, from boulders of gneiss in certain altered sediments of the central Alps, the conclusion has been drawn, without any attempt to establish the identity of the two kinds of gneiss petrographically, that the sediments were deposited upon the gneiss-like granite after its intrusion. The sediments were undoubtedly deposited upon some kind of a foundation, and it is probable that this foundation was composed, in part at least, of gneiss-like rocks. It cannot be questioned, however, that the gneiss-like granite is younger than the schist which it has penetrated and lifted. While rocks having the chemical compositions of argillites and sandstones indicate the previous weathering and denudation of a region of granite or gneiss just as clearly as do included boulders, yet one is not justified in concluding from the occurrence of sandstone or arkose in the beds overlying a granite that they were formed from the weathering of the latter. In conclusion, the results of these observations may be sum- marized as follows: 1. The crystalline schist formation is not a formation in a geologic sense, for certain more recent rocks have all the petro- graphic characters of the crystalline schists. Their petrographic similarity, which is the only reason for placing them all in the so-called Archean, can not be used, therefore, to determine their geologic age. The rocks which at the present time are classed as older or true crystalline schists, and are placed in the Archean formation, may be of any geologic age; the only property common to all being the fact that, as yet, no fossils have been found in them, the younger crystalline schists differing from them only in this respect. 2. The crystalline schist formation is not a formation in a petrographic sense, its rocks having different modes of origin. 180 FUNDAMENTAL PRINCIPLES OF PETROLOGY They are in part sediments, in part igneous rocks; some^ are primary, some are metamorphosed. 3. The so-called crystalline schists are not necessarily meta- morphic rocks, for some of them have not been metamorphosed. 4. Many extensive areas of crystalline schists are made up of igneous and contact-metamorphosed rocks of very different geologic ages. Instead of calling the different members mica- schist, etc., names which give false impressions as to their ages, they should be designated by proper descriptive names. Only after being thus separated will it be possible to determine the genetic relationships of true crystalline schists. 5. Dynamic disturbances primarily produce brecciation of the constituents of a rock. Whether they may give rise also to mo- lecular re-arrangements cannot be determined definitely. Most of our knowledge at the present time seems to indicate the contrary. 6. The theory of dynamometamorphism can no more withstand a critical examination than can the theories of the formation of primary crystalline schists, diagenesis, or plutonic or hydro- chemical regional metamorphism. 7. The oldest known fossiliferous sediments were formed under conditions which differed at most but very slightly from those existing at the present time. From the first formation of the earth to the deposition of the Cambrian, therefore, much more time must have elapsed than is generally allowed by geologists. The Cambrian must have been preceded by long periods of sedi- mentation, but no more traces of these deposits have been found than of the still older chemical precipitates from the hot, primordial universal sea. X. JOINTING AND TEXTURES LITERATURE F. BERWERTH: " Mikroskopische Strukturbilder der Massengesteine." Wien, 1895- 1900. CROSS, IDDINGS, PIRSSON, AND WASHINGTON: " The Texture of Igneous Rocks." Jour. Geol., XIV (1906), 692. J. P. IDDINGS: "The Crystallization of Igneous Rocks." Bull. Phil. Soc. Washington, XI (1889), 65. A. MICHEL-LEVY: "Structure et classification des roches eruptives." Paris, 1889. H. ROSENBUSCH: "tlber das Wesen der kornigen und porphyrischen Struktur bei Massengesteinen." Neues Jahrb., 1882, II, 11. IDEM: "tjber Struktur und Klassifikation bei Eruptivgesteinen." Tscherm. min. petr. Mitt., XII (1891), 351. W. SALOMON: "tJber Gestemskliiftung und Kliiftbarkeit." Der Steinbruch, VI (1911), 227. A. SAUER: "Mikroskopische Strukturbilder wichtiger Gesteinstypen." Stuttgart, 1906. H. C. SORBY: "On the Microscopical Structure of Crystals Indicating the Origin of Minerals, and Rocks." ^Quart+J'our. Geol. Soc., XIVv(1858), 433. F. ZIRKEL: "Mikrpskopische Struktur der Gesteine." Pogg. Ann., CXIX (1863), 288. + Appearance of Surface Exposures of Various Rocks. The general character of the'" surf ace exposures of any type of rock depends primarily upon its jointing and its texture. A rock mass is usually divided into separate parts by joint-planes which may be open or, in fresh rocks, hardly visible. Entering the rock along these joints, the agents of weathering proceed to destroy it, the amount and rapidity of the destruction depending primarily upon the texture of the rock and the number of joints. The relief of the surface is not produced exclusively by these factors, however, for the various climatic zones of weathering are of great importance, and one and the same rock may assume quite a different appearance in a different zone. Granular igneous rocks, which undergo chemical weathering relatively easily and are con- verted into grush by partial solution, show soft, rounded outlines with gentle slopes in regions in which chemical weathering pre- dominates. This is the normal mode of occurrence of granite in moist temperate or warm climates (Fig. 106). But the same rock takes on an entirely different form in arid regions. Here in 181 182 FUNDAMENTAL PRINCIPLES OF PETROLOGY many cases it is exposed in vertical walls (Fig. 107), and presents a very impressive appearance. There is a similar difference FIG. 106. Rounded exposures of granite with a rock-sea in the foreground. Grosse Schneegrube, Riesengebirge-. (H. Eckert, Prag, Photo.) between argillites, sandstones, etc. under these conditions, for while they disintegrate readily in humid regions and rarely out- FIG. 107. Granite walls, Mount Sinai. (After v. Lendenfeld.) crop, in arid regions they stand out in most grotesque shapes as hoodoos, buttes (Fig. 51), and other bad-land forms. JOINTING AND TEXTURES 183 Rocks which have no regular joint-planes but are quite compact, especially serpentine (Fig. 108), reef-limestones and dolomites (Fig. 55), quartz-dikes (Fig. 56), etc. tend under all conditions to stand out in distinct relief from their surroundings. FIG. 108. Serpentine stock. Goslerwand, Gross Venedig province. But it is not alone the firmly-cemented and jointless rocks which appear in steep cliffs; these forms are just as characteristic in mas- sive deposits of loose, fine-grained materials which weather readily. FIG. 109. Trass deposits in the Brohltal near Andernach on the Rhine. Examples may be found in the cliffs of the Appenines, along the chalk shores of the Baltic and North Seas, in the trass deposits in the Brohl Valley (Fig. 109), and in the loess deposits of central 184 FUNDAMENTAL PRINCIPLES OF PETROLOGY Asia. Just as characteristic are the so-called earth pillars or hoodoos (Fig. 63) which are found in many porous glacial deposits, talus slopes, and volcanic tuffs. If the characteristic forms of rocks which possess distinct joint- ing are examined, it will be seen that there is a close relationship between these forms and the forms of the mountains themselves. Where normal granites have approximately horizontal joint- planes, they appear in the usual rounded exposures, but where the schistosity is good and is at a high angle, the rock appears in sharp ridges and ragged peaks (Fig. 110). ,Various sedimentary rocks, FIG. 110. Jagged peaks in schistose granite (protogine). Aiguilles des Charmoz et de Trelaporte. (Wehrli, Zurich, Photo.) here and there, show similar ragged features when their bedding- planes are steeply inclined. Where the strata are more nearly horizontal the exposures are flat, and the planes may be deeply dissected by canyons (Fig. 53). Jointing and Parting in Rocks. The terms jointing and parting are applied to certain directions in rocks in which they fracture more easily than in others. In this sense joint-planes include the bedding-planes of sedimentary rocks as well as certain fracture- planes, called rift and grain by quarrymen, of many igneous rocks. The latter lines of weakness, which may be imperceptible to the inexperienced in rocks which are fresh but distinctly visible when JOINTING AND TEXTURES 185 they are weathered (Figs. 6 and 47), permit the removal of rec- tangular blocks with relatively plane faces from quarries in which the rocks appear to be perfectly massive. The stresses producing joints of this kind were developed by the contraction of the cooling igneous rock. They must have been of extraordinary force at times, for the hardest and least cleavable minerals, such as quartz and olivine, have in some cases been so perfectly and smoothly sheared that the two parts of a crystal may be found in opposite sides of a fissure. The most common method of separation in igneous rocks is the so-called platy-parting, which predominates in persilicic and mediosilicic rocks of the granite, quartz-porphyry, phonolite, and FIG. 111. Platy-parting laccolith. Flossenbiirg, near Weiden, Oberpfalz. andesite groups. This parting is generally parallel to the cooling surface; in granite (Fig. Ill), for example, it is parallel to the surface of the laccolith, while in extrusive rocks, such as phonolite, it is parallel to the surface of the lava-flow. In vertical dikes, the parting is vertical, and the joints, parallel to the cooling sur- face, may give rise to gorges with vertical walls, as in the Eggental (Fig. 112). The behavior of different rocks with platy-parting, when sub- jected to weathering, is quite different. The plates of fissured granites are only slightly thinner near the surface than farther down, while phonolite, under the ame conditions, splits into plates no more than a millimeter in thickness, the so-called paper- 186 FUNDAMENTAL PRINCIPLES OF PETROLOGY porphyry (Fig. 49). Plates of quartz-porphyry generally differ but little, whether fresh or weathered. FIG. 112. Platy -parting in quartz-porphyry. Eggental, Bozen. Second in importance is columnar jointing. This occurs primarily in subsilicic extrusive rocks such as trap (Fig. 113), FIG. 113. Prismatic parting in trap. Giant's Causeway, Ireland. (After F. Toula.) melaphyre, and basalt, but it is also found in silicic rocks, for example in the quartz-porphyry from Sigmundskron, near Bozen. JOINTING AND TEXTURES 187 Upon weathering, such columns develop cross-joints, and gradually exfoliate into onion-like spheres (Fig. 48). In other cases oblique FIG. 114. Oblique parting in quartz-porphyry. Crest of the Wolf stein, Kosten, Bohemia. (Eckert, Frag, Photo.) parting-planes cause the rock to break into acute-angled blocks (Fig. 114). t^^-^T^*^ FIG. 115. Perlitic parting. Perlite. Glashuttental, Schemnitz, Hungary. Glassy igneous rocks, in many cases, show spheroidal partings, varying from the mi- croscopic pearl-like shells in perlite (Fig. 115) to spheres as large as one's fist in kugel- porphyry. These cracks are recognizable even in completely weathered material, while in fresh rocks the parting may be so complete that the rock falls into a grush called marecanite. Here and there distinctly crystalline igneous rocks, such as mela- phyres, show a coarse spheroidal parting. 188 FUNDAMENTAL PRINCIPLES OF PETROLOGY Sedimentary rocks may also show a parting independent of the bedding. Thus the parallelopipedal parting of sandstone is caused by two very perfect cleavages at right angles to the bedding. Rocks which are so joined break up, under the influ- ence of weathering, into grotesque columns, such as those which make the scenery of Saxon Switzerland so attractive (Fig. 116). FIG. 116. Parallelopipedal parting in sandstone. Adersbach, Bohemia. (H. Eckert, Prag, Photo.) A great deal of parting is due to secondary orogenic forces. Thus most of the gneiss of the Bavarian Forest is fractured across the schistosity, while certain indistinctly schistose granites of the Oberfalz are so regularly fissured and recemented by secon- dary biotite,that the rock appears to be schistose and banded in two directions (Fig. FIG. 117. Granite. Wondreb, Oberpfalz. 117. In the figure the secondary cleavage, with its biotite-filling, is horizontal). Similar, but much more uniform, is transverse schistosity or rock cleavage. This is developed at right angles to the pressure, and cuts the bedding of sedimentary rocks at an angle. It is generally much smoother and more complete than the bedding- plane itself. Such cleavage is found in many sandstones, but it is especially well JOINTING AND TEXTURES 189 developed in certain argillites (Fig. 118), which become very compact and are then of commercial importance as roofing-slate. Under the same forces, brittle rocks of uniform texture, such as quartzites or FIG. 118. Transverse schistosity cutting across somewhat bent beds of slate. Goslar. (Dr. Baumgartel, photo.) dolomites, are crushed to small angular fragments, in many cases quite uniform in size. These are later recemented and form the so-called endogenic (Gr. tvSov, within) breccias (Ital. breccia, break) (Fig. 119). If the cement weathers easily, these rocks FIG. 119. Dolomite with parallel systems of veins. Saalburg. break down into an angular grush, such as is characteristic of numerous Alpine dolo- mites. Parting may also be brought about by the heat of contact-metamorphism. This is shown in the columnar parting of fritted sandstones, granites, burnt clays, and coked coals. 190 FUNDAMENTAL PRINCIPLES OF PETROLOGY Megascopic Characters of Rocks. The appearance of a rock, that is its habit, depends upon the development and arrangement of its constituents so far as these can be seen megascopically. In certain rocks, called phanerites (Gr. Qaveds, distinct), the chief constituents, and usually their arrangement and development, can be distinctly recognized with the unaided eye or by the aid of a low-power hand-lens. In other rocks the greater part of the con- stituents cannot be seen without the microscope. Such rocks are megascopically cryptomerous (Gr. KPUTTTOS, hidden) or aphanitic (Gr. d, out), and consist of fragments of the country-rock, torn loose by the igneous magma. In many plutonic rocks these have been so far assimilated that they stand out from the otherwise homogeneous rocks only as poorly defined spots, the so-called basic inclusions. Gneiss inclusions in granite may have preserved their original schistose character and have become filled with injections of the igneous rock. Inclusions are especially numerous in small dikes and in the border-zones of large intusives where they form contact- breccias (Fig. 134). A great variety of exogenic inclusions occur in volcanic tuffs. They consist of normal contact-rocks, which were clearly altered before the magma reached the sur- face, and fragments of the country-rocks through which the rising magma passed. The latter are usually fritted. Especially interesting to the mineral collector are the FIG. 137. Agate, showing conduit. Oberstein a. N. inclusions and ejectamenta of sodic rocks, which in many cases are entirely saturated by the constituents of the magma. Concretions (Lat. concretus, grown together) are concentrations of certain constitu- ents in sediments. In some cases they originated during the rock formation, in others by later processes. Here belong the clay-pockets of sandstones, the much-fissured, lens-like septaria of clays, nests of gypsum, pyrite, marcasite, and siderite hi marls and argillites, of flint in chalk, and of menilite in the siliceous schists; also loess-kindl (Fig. 135) representing calcareous concretions, sand-filled -calcite-clusters in the so- called crystallized sandstone (Fig. 136), knots and bands of hornstone and carnelian in limestones of different formations, and finally masses of limonite, phosphorite, celestite, etc. Secretions (Lat. secretus, separated), or more correctly infiltrations, include all cavity-fillings in rocks. Among these are amygdules and geodes of calcite, zeolites, and agate (Fig. 137) in vesicules of igneous rocks, veins of quartz and calcite in all kinds of rocks, and all mineral aggregates in cavities of any kind. The material in all cases was brought in from external sources. ^ '"T^.* V .^t. PLATE II. H-' ; IBs ^$ji.-*->3& FIG. 1. Granitic texture. (After Berwerth.) The dark constituents show distinct boundaries against the light. Quartz (the lightest mineral in the figure) was the last mineral to crystallize. It fills the interstices between the earlier con- stituents. Ordinary light. FIG. 2. Granulitic texture. Quartz shows distinct boundaries against the re- maining constituents. Polarized light. FIG. 3. Micropegmatitic texture. The eutectic mixture of orthoclase and quartz in graphic intergrowth was the last product of crystallization. Polarized light. FIG. 4. Monzpnite texture. The or- thoclase fills the interstices between the plagioclase laths. Polarized light. FIG. 5. Gabbro texture. Irregular, granular arrangement. Polarized light. FIG. 6. Ophitic texture. Large au- gite individuals fill the spaces between plagioclase laths. Ordinary light. PLATE III. FIG. l.Porphyritic texture. (After FIG. 2. En taxi tic texture. The Berwerth.) The groundmass is micro- groundmass shows distinct fluidal texture, granitic, and in this lie large phenoerysts Ordinary light, of quartz and feldspar. Polarized light. FIG. 3. Spherulitic texture. The groundmass consists predominantly of radially arranged quartz-feldspar aggre- gates. Polarized light. FIG. 4. Trachytic texture. (After Berwerth.) Feldspar microlites are ar ranged in flow-lines around the pheno- crysts. Polarized light. FIG. 5. Hyalopilitic texture. The groundmass is rich in glass^and contains unoriented microlites of plagioclase and augite. Ordinary light. ./A FIG. 6. Intersertal texture. (After Berwerth.) The last remnant of the magma solidified as a microlite-filled glass. It fills the interstices between the remain- ing constituents. Ordinary light. PLATE IV. FIG. 1. Sieve texture. A garnet crystal with poor boundaries is filled with inclusions of the other constituents. Or- dinary light. ggre- gate of pyroxene and plagioclase. Polar- ized light. FIG. 3. Helizitic texture. Biotite, sillimanite, and ilmenite crystals, ar- ranged parallel to the original bedding, intersect a crystal of cordierite which takes up the greater part of the field of view. Polarized light. FIG. 4. Mosaic texture. The albite crystals are arranged as in a mosaic. Polarized light. FIG. 5. Biolite with its long direc- tion at right angles to the cleavage of the rock. Ordinary light. FIG. 6. Bent biotite lamellae. Though the mica is greatly bent, it shows no breaks. Polarized light. PLATE V. FIG. 1. Mosaic texture. Quartz grains with rather plane contacts. Polar- ized light. FIG. 2. Sutured texture. The quartz individuals interlock and are firmly united. Polarized light. FIG. 3. Sutured texture in itacolu- mite. The quartz individuals interlock but are not firmly united . Ordinary light. FIG. 4. Mortar or murbruk texture. Shows fine quartz grains in fissures in quartz grains with undulatory extinction. Polarized light. FIG. 5. Clastic texture. Angular quartz "grains, clearly alluvial, in a dense, clay-like cement. Polarized light. FIG. 6. Clastic texture. Rounded quartz grains, clearly aeolian, with rims of secondary quartz. Ordinary light. PLATE VI. FIG. 1. Mosaic texture. Calcite crystals with rather plane contacts. Polarized light. FIG. 2. Sutured texture. The calcite grains interlock. Polarized light. FIG. 3. Cataclastic texture. Fine calcite aggregates between strained rem- nant* of larger grains of calcite. Polar- ized light. FIG. 4. Mechanical texture. The twinning lamellae of the calcite are bent yet the mineral is not broken. Ordinary light. FIG. 5. Oolitic texture. Show? the original radial and concentric arrange- ment within the individual spherules. Polarized light. FIG. 6. Oolitic texture. The spher- ules have been altered by diageresis to a granular aggregate. Polarized light. INDEX Abrasion, 89 Abyssal differentiation, 48 rocks, 17 Accessory minerals, 31 Acid rocks, 33 Adiagnostic rocks, 190 Adinole, 137 Aeolian sediments, 95, 97 Agate, 145, 205 Age classification of igneous rocks, 28. of igneous rocks, 27 of Catastrophies, 20 Agents of contact-metamorphism, 116 Allothigenic constituents, 6 Allotriomorphic, 198 Alluvial sediments, 95, 99 Alluvium, 85 Alpine granite, anomalous character, 59 Alps, origin of border zone, 135 Alunitization, 154 Amygdaloids, 144, 191 Amygdule, fillings, 145 Amygdules, 205 Anamorphism, 78 Andernach, 44 Andesitic texture, 199 Anogenic metamorphism, 164 rocks, 7 Aphanitic rocks, 190 Aplite related to granite, 22 Aplites, 5, 49 never occur in unmetamorphosed sediments, 23 Aplitic injections, 121 texture, 198 Apophyses, 28 Aragonite formation, 103 Argillites, 96 contact-metamorphism of, 123 Aschaffenburg, 52 Aschistic rocks, 49 Ashes, volcanic, 25 Atmosphere, 6 Augen-gneiss, 56 Authigenic constituents, 6 Automorphic, 197 Average composition of earth, 34 B Bad-land topography, 89 Banded hornfels, 63 -gneisses, 66 rocks, 41, 191 Basic igneous rocks, contact-metamorph- ism of, 131 rocks, 33 Basis, 17 Bauxite, 79, 83 Bavarian Forest, 88 Bedded rocks, 7, 194 Bedding, cross, 98 diagonal, 98 direct, 101 indirect, 101 Bennan Head, Arran, 67 Bioliths, 1.06 Bitumen, formation of, 110 Bituminous coal, 109 Black Hills, S. D., 88 Blastogranitic texture, 200 Blastophitic texture, 201 Blastoporphyritic texture, 201 Blastopsammitic texture, 201 Blastopsephitic texture, 201 Block-lava, 26 Blue-mud, 101 Bombs, 25 Borax-seas, 80 Bozen, South Tyrol, 44 Breccia, endogenic, 189 Brogger's diagrams, 71 Brohl valley, 44 Brown coal, 109 Bunsen's theory of differentiation, 54 Calcareous sinter, formation of, 104 Calciphyres, 128 207 208 INDEX Calcite formation, 103 Carbonaceous deposits, 108 Carbonate rocks, contact-metamorphism of, 127 Carbon dioxide from volcanic eruptions, 84 Cataclastic texture, 203 Catamorphism, 78 Catogenic rocks, 7 * Caustobioliths, 106 Cave formation, 75 Cavernous rocks, 191 Cellular rocks, 191 Center of earth, condition of, 8 Chelif River, Algeria, 80 Chemical composition of igneous rocks, 34 sediments, 95 weathering, 74 formerly, 83 Chief minerals, 31 Chilian pampas, 80 Chlorite-schists, 132 Christiania basin, Norway, differentia- tion in, 54 Classification of crystalline schists, 162 Clastic rocks, 6 Clay, 77 Clay-pockets, 205 Clay-slate needles, 96 Climatic zones of weathering, 82 Coal, 108 method of formation, 109, 113 Coarse-grained rocks, 190 Columnar jointing, 186 Comagmatic region, 19 Complementary dikes, 48 Composite dikes, 53 Composition of igneous rocks, 31 of volcanic emanations, 15 Compound rocks, 5 Concretions, 204, 205 Conglomerates, 96 Consanguinity, 53 Constitution schlieren, 44 Contact-limestone minerals, 136 Contact-metamorphic agents, 116 Contact-metamorphism, 116 by extrusive rocks, 137 by plutonites, 119 of argillites, 123 of basic igneous rocks, 131 of carbonate-rocks, 127 susceptibility of a rock to, 121, 124 Coral reefs, 106 Corrasion, 89 Cross, Iddings, Pirsson, and Washing- ton's system, 72 Cross-bedding, 98 Cryptogenic rocks, 7 Cryptomerous rocks, 190 Cryptovolcanic activity, 27 . Crystalline rocks, 5 schists, 7, 157 action of granite on, 176 characteristics of, 11 classification of, 162 early ideas regarding, 157 facies of, 173 origin of, 179 thickness of, 177 variability of, 160 younger, 12, 158 Crystallization-schistosity, 200 Crystallization sequence, 197 Crystallized sandstone, 205 Crystalloblastic texture, 200 Crystalloids, 77 Cyclopic texture, 200 Deep-seated rocks, 17, 24 Deflation, 89 Dense rocks, 190 Denudation, 88 Desmosite, 138 Devil's wall, Bohemia, 93 Diabase-hornfels minerals, 136 Diablastic texture, 200 Diagenesis, 74, 111, 161 Diagonal-bedding, 98 Diaschistic rocks, 49 Diatremes, 21, 27 Differentiation, abyssal, 48 dikes, 49 laccolithic, 48 magmatic, 44 schlieren, 44 theories, 54 Dike rocks, 49 Dikes, 24 composite, 53 double, 53 on the Isle of Arran, 93 welded, 52 INDEX 209 Direct bedding, 101 Dislocation metamorphism, 164 Dolinas, 75 Dolomite-ash, 130 Dolomitization, 113 Double dikes, 53 Dreikanter, 89, 98 Dynamometamorphism, 56, 164, 169 in Alps, 135 E Earth pillars, 102 Earth's crust, formation of, 9 Eclogite, 132 Effusive rocks, 17, 25 Elements, distribution of, 33 Endogenic breccias, 189 contact phenomena, 47 inclusions, 204 Eozoon, 12, 159 Epi-rocks, 174 Erosion, 89 Eruptions, explosive, 27 quiet, 27 Erzgebirge, 48, 61 Essential minerals, 31 Eutaxitic groundmass, 198 Eutectic intergrowth of calcite and dolo- mite, 159 mixtures, 38 Exaration, 89 Exogenic inclusions, 205 Explosive eruptions, 27 Explosiveness of magma, 22 Extrusive rocks, 17, 25 contact-metamorphism of, 137 Facies of crystalline schists, 173 of granite, 45 Felsophyric, 198 Fichtelgebirge, 63, 86, 87 Fine-grained rocks, 190 Fission algae, 113 Fleckschiefer, 126 Fluctuation texture, 191 Fluidal texture, 191 Fluviatile sediments, 99 Folding, fractureless, 165 Forms of weathering, 85 Fossil sediments, 114 Fractureless folding, 165 Fragmental rocks, 6 Frankland, 81 Fresh-water limestones, formation of, 104 Friction-breccias, 170 Fritted rocks, 137, 189 Frothy rocks, 191 Fruchtschiefer, 126 G Gabbroic texture, 198 Ganggefolgschaft, 49 Garbenschiefer, 126 Garben texture, 199 Gases from volcanoes, 84 Gels, 6, 77 Geodes, 205 Geologic pipe organs, 75 Giant-grained rocks, 190 Glacial sediments, 95 Glashiittental, Schemnitz, 42 Glassy groundmass, 198 rocks, 6 Glauconite-sand, 101 Gossan, 142 Grain, 87, 184 Granitic texture, 39, 197 Granoblastic texture, 200 Granoblasts, 176 Granophyric groundmass, 198 Granular rocks, 17, 197 Granularity of dikes, 48 Granulitic texture, 39, 198 Graphical representations of composi- tion, 68 Graphite deposits, 147 Gravel, 96 Green-mud, 101 Greensand, 101 Greenstones, formation of, 151 Greenstone-schist, 131 Greisen, 146 Grus, 80 Grush, 80 Gumbel's age classification, 28 theory of diagenesis, 161 Gypsum chimneys, 75 formation of, 104 H Haphazard texture, 46, 191 Hawaiian lava temperature, 39 Hebrides, 42 210 INDEX Helm's theory, 169 Helizitic texture, 118, 200 Henry Mountains, Utah, 64 Herculaneum, 21 Holocrystalline rocks, 197 Holocrystalline-porphyritic texture , 198 Homogeneous volcanoes, 26 Homooblastic texture, 200 Koodoos, 103 Hornfels, 125 banded, 63 Hornschiefer, 126 Hutton, 66 Hyaline rocks, 6 Hyalopilitic texture, 199 Hybrid rocks, 135 Hydrochemical metamorphism, 164 Hydrochloric acid from volcanic erup- tions, 84 Hydrosphere, 6 Hypidiomorphic-granular texture, 197 Hypocrystalline-porphyritic texture, 198 Hysterocrystallization, 37 Hysterogenic schlieren, 44 Jekaterinburg, Urals, 39 Jointing, 86, 184 columnar, 186 Juvenile waters, 18 K Kant, 14 Kaolin formation, 76 Kaolinization, 149 Karst topography, 75 Katamorphism, 78 Kata-rocks, 174 Katogenic metamorphism, 164 Kaulquappenquarze, 168 Kidney marble, 195 Kilauea, Hawaii, 27, 44 Klingenberg, 81 Klosterberg, 27 Kneaded texture, 171, 204 Knoten texture, 199 Knotenglimmerschiefer, 126 Knotenschiefer, 124, 126 Iceland, 42, 54 Idioblastic texture, 200 Idiomorphic, 197 Igneous rocks, 7 age, 27 Implication texture, 198 Included constituents, 32 Inclusions, 204 endogenic, 204 exogenic, 205 metamorphism of, 138 primeval, 52 Indirect bedding, 101 Injected-schist, 48 Injection rocks, 61 -schist, 48 -schlieren, 44 Insolation, 74 Intersertal texture, 199 Intratelluric phenocrysts, 17, 42, 198 Intrusive rocks, 17, 24 Isar, 58 Isle of Skye, 47 Laccoliths, 24 Laminated rocks, 191 Lamprophyres, 49 Lapilli, 25 Laplace, 14 Large-grained rocks, 190 Latent plasticity, 165 Laterite, 78, 83 Lava sheets, 26 streams, 26 Laws of association for contact-meta- morphism, 137 of minerals in igneous rocks, 33 Leidenfrost phenomenon, 15 Lepidoblastic texture, 200 Leucocratic, 48 Liquation, 39, 65 Literal deposits, 100 Loess, 98 Low Tauern, 59 Luxullianite, 146 Lydite, 138 INDEX 211 M Magma, 18 a complex solution, 22 basins, 19 explosiveness of, 22 physical character of, 21 physico-chemical laws of, 36 Maine, 52 Marecanite, 187 Marine sediments, 99 Marls, 97 Martinique, 16, 21, 22, 27 Massive rocks, 7 Mechanical sediments, 95 textures, 202 Mediosilicic, 33, 35 Medium-grained rocks, 190 Melanocratic, 48 Meso-rocks, 174 Mesostasis, 197 Metagneisses, 163 Metamorphism, anogenic, 164 dislocation, 164 dynamo-, 164, 169 hydrochemical, 164 Neptunian, 164 Plutonic, 164 Meta-rocks, 173 Metasomatic, 155 replacement of carbonate rocks, 155 Miarolitic rocks, 191 Michel-Levy's formulae, 68 Microgranitic, 198 Microlitic texture, 199 Micropegmatitic texture, 198 Migmatites, 135 Mineral laws of association, 33 Mineral-dikes, 146 Mineralizers, 18, 22, 164 action of, 41 reduce viscosity, 23 Minerals, accessory, 31, 32 chief, 31 essential, 31, 32 included, 32 primary, 32 secondary, 32 substitute, 31, 32 unessential, 31 Mixed rocks, 5, 135 Mixed-type, 67 Mont Blanc fan structure, 61 Monzonitic texture, 197 Mortar texture, 203 'Mosaic texture, 200 Murbruk texture, 203 Mylonites, 170, 204 N Nebulite, 65 Xematoblastic texture, 200 Neptunian metamorphism, 164 Neubildungen, 76, 83 Oberpfalz, 62 Oblique parting, 187 Odenwald, 46, 64, 86 Oolitic texture, 202 Ophitic texture, 198, 200 Ore-veins, 146 Organic weathering, 74, 84 Organogenic sediments, 80, 95 Origin of border zone of Alps, 135 Orthogneisses, 163 Orthophyric texture, 199 Ortho-rocks, 173 Osann's classification, 68 diagrams, 72 Palaopicrite, 28 Palimpsest texture, 118, 196, 200 Panidiomorphic texture, 198 Panzerdecke, 19 Paper-porphyry, 185 Paragenesis of contact-rocks, 136 Paragneisses, 163 Para-rocks, 173 Parting, 86, 184 oblique, 187 parallelopipedal, 188 perlitic, 187 spheroidal, 187 Passau, 65 Pegmatite, formation of, 142 minerals of, 143 Pegmatitic texture, 193 Pelites, 96 Penkatite, 130 212 INDEX Perlitic parting, 187 Persilicic, 33, 35 Petrographic character and geologic age, 13 province, 19, 53 Petroleum, formation of, 110, 114 Petrology, definition, 1 Phanerites, 190 Phenocrysts, 17 Physical weathering, 74 Physico-chemical laws of the magma, 36 Phytogenic deposits, 107 Piezo-contact-metamorphism, 134, 177 Piezocrystallization, 55 Pilotaxitic texture, 199 Pisolite, formation of, 104 Pitchstone contains water, 18 Plankton, 106 Plasticity, latent, 165 Platy-parting, 185 of phonolite, 87 Plutonic metamorphism, 164 rocks, 17 Pneumatohydatogenic processes, 139 Pneumatolitic-metamorphism, 66 Pneumatolitic processes, 139 Poikilitic texture, 198 Poikiloblastic texture, 200 Pompeii, 21 Porcellanite, 137 Porodine deposits, 6 Porous rocks, 191 Porphyritic rocks, 17 texture, 193, 200 Porphyroblastic texture, 200 Porphyroblasts, 176 Porphyroclastic texture, 201 Post-volcanic processes, 139 Pre-Cambrian South African sediments, 12 Predazzite, 130 Primary minerals, 32 rocks, 95 Primeval inclusions, 52 Propylite, 148 Propylitization, 148 Proterobase^S Protoclaatic texture, 203 Protogine, 135, 172 Psammites, 96 Psephites, 96 Pseudoporphyritic texture, 199 Putzen, 44 Pyriboles, 32 Q Quellkuppen, 26, 44 Quiet eruptions, 27 II Random texture, 191 Recent sediments, 114 Regional metamorphism, 156, 162 Relation of folding to intrusion in Alps, 59 Replacement, 73, 140 Resorption 37 of phenocrysts, 17 -schlieren, 44 Richtungslose, texture, 46 Riecke's principle, 56, 170 Ries, Bavaria, 44] Riesenberg, Bohemia, 63 Rift, 87, 184 Ripple-marks, 98 Roches moutonnees, 91 Rock, average, 34 -sculpture, 85 -seas, 87 -streams, 87 weathering, 73 Rocks, acid, 33 abyssal, 17 adiagnostic, 190 anogenic, 7 aphanitic, 190 basic, 33 bedded, 7 catogenic, 7 chemical composition of, 34 clastic, 6 compound, 5 cryptogenic, 7 cryptomerous, 190 crystalline, 5 deep-seated, 17, 24 definition of, 5 effusive, 17 extrusive, 17 fragmental, 6 fritted, 137 glassy, 6 INDEX 213 Rocks, granular, 17 hyaline, 6 hybrid, 135 igneous, 7 intrusive, 17 massive, 7 mixed, 5, 135 plutonic, 17 porphyritic, 17 primary, 95 secondary, 95 sedimentary, 7 simple, 5 surface, 17 Rosenbusch's graphical method, 71 kern theory, 55 Rubble, 96 Rutile needles, 96 s Salt, formation of, 104 -pans, 80 -eelites, 99 Sand, volcanic, 25 Sandstones, 96 Satellite dikes, 48 Saussurite hornfels minerals, 136 Saussuritization, 151 Scaradrapass, 56 Schistes feldspatises, 67, 119 Schistose rocks, 191 Schists, crystalline, 7 Schlieren, 39, 44 constitution-, 44 resorption-, 44 injection-, 44 differentiation-, 44 hysterogenic-, 44 Schlossberg, Bohemia, 88 Scoriaceous rocks, 191 Secondary minerals, 32 rocks, 95 Secretions, 204, 205 Sedimentary rocks, 7 Sediments, seolian, 95, 97 alluvial, 95, 99 chemical, 95, 103 composition of, 95 fluviatile, 99 fossil, 114 glacial, 95, 102 Sediments, marine, 99 mechanical, 95 organogenic, 80, 95, 106 recent, 114 Septaria, 205 Sequence of crystallization in granite, 197 of minerals, 37, 38 Sericitization, 152 Serpentinization, 152 by contact-metamorphism, 132 Sheets, 24, 26 Sieve texture, 118, 199 Silicification, 154 Simple rocks, 5 Skarn, 129 Slate-hornfels minerals, 136 Soil-zeolites, 77 South Africa unfossiliferous sediments, 12 Specific gravity of earth, 8 of earth's crust, 8 Spheroidal parting, 187 texture, 193 weathering of basaltic columns, 88 of diabase, 87 Spherulitic groundmass, 198 Spilosites, 138 Spring's experiments, 169 Stalactites, formation of, 104 Steinheimer Basin, 27 Stellvertreter, 31 Stigmaria, 109 Stink-stone, 130 Stocks, 24 Strato-volcanoes, 26 Streams, lava, 26 Stretched sandstone, 168 Stiibel's theory, 19, 178 Subsilicic, 33, 35 Substitute minerals, 31 Surface rocks, 17 Sutured texture, 200 Swabian Alb, 21, 44 Tad-pole quartz, 168 Talc formation, 153 Temperature at earth's center, 8 gradient, 8 within the earth, 14 . Terra rossa, 75 Texture, andesitic, 199 214 INDEX Texture, aplitic, 198 blastogranitic, 200 blastophitic, 201 blastoporphyritic, 201 blastopsammitic, 201 blastopsephitic, 201 cataclastic, 203 crystalloblastic, 200 cyclopic, 200 diablastic, 200 fluctuation, 191 fluidal, 191 gabbroic, 198 garben, 199 granitic, 39, 197 granoblastic, 176, 200 granular, 197 granulitic, 39, 198 haphazard, 46, 191 helizitic, 118, 200 hole-crystalline, 197 homooblastic, 200 hyalopilitic, 199 hypidiomorphic-granular, 197 idioblastic, 200 implication, 198 intersertal, 199 kneaded, 171, 204 knoten, 199 lepidoblastic, 200 mechanical, 202 microlitic, 199 micropegmatitic, 198 monzonitic, 197 mortar, 203 mosaic, 200 murbruk, 203 nematoblastic, 200 oolitic, 202 ophitic, 198, 200 orthophyric, 199 palimpsest, 118, 196, 200 panidiomorphic, 198 pegmatitic, 193 pilotaxitic, 199 poikilitic, 198 poikiloblastic, 200 porphyritic, 193, 198, 200 porphyroblastic, 176, 200 porphyroclastic, 201 protoclastic, 203 pseudoporphyritic, 199 Texture, random, 191 richtungslose, 46 sieve, 118, 199 spheroidal, 193 sutured, 200 trachytic, 198 xenoblastic, 200 Textures of contact-rocks, 199 of crystalline schists, 199 of igneous rocks, 196 of sedimentary rocks, 201 Thermal processes, 139 Thiiringen, 48 Trachytic texture, 198 Tuff, 26 Two generations of crystals, 17 Type-mixing, 66 W Washakie Basin, Wyoming, 90 Waters, juvenile, 18 magmatic, 18 vadose, 18 Weathered residues, 80 Weathering, 73 chemical, 74 organic, 74, 84 physical, 74 solutions, 79. Webern, Odenwald, 46 Welded dikes, 52 Wener Sea, Sweden, 42 Werner's theory of vulcanism, 14 Wiesalpe, Dachstein, 75 Wiesbaden limestone, 7 X Xenoblastic texture, 200 Xenomorphic, 198 Yellowstone Park, 42 Younger crystalline schists, 158 Zeolitization, 154 Zillertal, 133 Zoogenic limestones, 106 Zwitter, 146 THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO SO CENTS ON THE FOURTH .DAY AND TO $1.OO ON THE SEVENTH DAY OVERDUE. -.- 193 5*$ WAY 24183 3-0- la 1S58- NOV 1 1938 DEC I r ^1939 tf* LD 21-95m-7,'37 YC 40244 UNIVERSITY OF CALIFORNIA LIBRARY